Man

Neudeckian" (Third Interglacial), (Fourth Glacial), Mecklenburgian "Lower Forestian" glacial), "Lower Turbarian" (Fourth InterForestian" (Fifth Interglacial), and (Fifth Glacial), "Upper (Sixth Glacial) Epochs. Upper Turbarian

64

According to the terminology usually adopted by British geologists, the Glacial Period came to an end with the final disappearance of the confluent ice-sheets from our lowlands, and the events which followed are classed as Postglacial. But the latter period has been sufficiently long to Cover some extensive changes in the relative distribution of land and sea in Western Europe, accompanied by modifications of climate tending on the whole toward progressive amelioration. To classify these changes into a further series of three interglacial and three glacial epochs, as Prof. J. Geikie has done, is, so far as the British evidence is concerned, mainly a question of personal opinion as to the arrangement of the sequence and the application of terms. As we have already seen, the interpretation of the North European sequence, on which Prof. Geikie greatly depends for proof of these later epochs of glaciation, has been challenged abroad even by geologists favourable to the general principle of interglacial epochs; and we are, therefore, the more fully entitled to question its application in this country.

In Scotland, Prof. Geikie claims that the "Mecklenburgian glaciation was marked by the reappearance of glaciers in the mountain valleys, and by their later extension over part of the neighbouring lowlands in the form of district ice-sheets. away during the "Lower Forestian After these had melted there is supposed to have been a regrowth of valley-glaciers interglacial time, that came down to sea-level during the "Lower Turharian" stage. Then another melting away marked the Upper Forestian," followed by a fresh appearance of glaciers in the glens of the higher mountain groups during Upper Turbarian" glacial epoch.

the "

seems

But all the phenomena on which this scheme is built seem explicable on the hypothesis of a gradually waning glaciation, during which there advances of the mountain-glaciers in their glens, due to were occasional local temporary increase of snowfall. cussed the probability that the growth of the individual We have already disice-sheets was largely influenced by the local impact of snowfall under changing meteorological conditions, and it equally probable that similar changes, in reverse order, accompanied the waning of the same sheets. Indeed, from the study of recent glaciers, it has been shown that the presence of separate moraines need not indicate separate stages of advance in the ice. ing the influence of englacial débris on ice-flow, the late In discuss"Report on the Character of the High-level Shell-bearing Deposits at Clava, Chapelhall. and other Localities." Clava), pp. 483-514: ibid. for 1894 (Chapelhall), pp. 307-315; ibid. for Rep. Brit. Assoc. for 1893 1996 (Kintyre), pp 178-399.

Mr. Jamieson's latest papers: "The Glacial Period in Aberdeenshire and the Southern Border of the Moray Firth," Quart Journ. Geol. Soc.. nl. Ixii. (1905), pp 13-19, and "On the Raised Beaches of the Geological Survey of Scotland," Geol. Mag., dec. v., vol. iii. (1906), pp. 22-25, contain an excellent descriptive summary and discussion of the glacial sequence.

NO. 1920, VOL. 74]

399

Prof. Israel C. Russell has the following pertinent remark: "The considerations . . . lead to the suggestion that a series of terminal moraines in a formerly glaciated valley, glacier, are not necessarily evidences of repeated climatic or a similar succession of ridges left by a continental oscillations, but may have been formed during a uniform and continuous meteorological change favourable to glacial recession. That is, a débris-charged sheet may retreat for becoming congested with foreign material, in response to a time, then halt, and again retreat, owing to its terminus a climatic change which would cause a glacier composed of clear ice to recede continuously and without halts."1 Prof. Geikie states his case for the " persuasiveness. He acknowledges that no interglacial dedistrict ice-sheets Mecklenburgian posits of the preceding Neudeckian epoch have been recogwith intrepid but unconvincing nised in Britain, and bases his argument upon the relation of the hill-drift to that of the lowlands. Into the intricacies of this argument it is impossible for me to enter, but there is one point which requires particular notice. The shelly boulder-clay around Loch Lomond is held to represent the Mecklenburgian glaciation, and its marine detritus to have been derived from a sea-floor belonging to the 100-foot raised beach, an early stage of the same glacial epoch. But, as Mr. " which is supposed to mark T. F. Jamieson has shown, there is no valid reason for regarding this boulder-clay as newer than the bulk of the shelly boulder-clays of Scotland; it rests directly upon the clay with shells was found beneath it; and no older solid rock, except at one place, where a wedge of blue original description of the deposit given by Dr. R. L. boulder-clay is known in the district. Jack, quoted with approval by Prof. Geikie, we Even from the gather no other interpretation; for although Dr. Jack thought that the shells were more probably derived from an interglacial than from a preglacial bed, he still regarded the boulder-clay in which they occur as older than glacial epoch of the new classification. the 66 great submergence "-i.e., than the Helvetian inter

4

3

can

46

66

to

hypothesis. In any case, it is not likely that many British geologists will be found willing to regard the hill peats as other than post-glacial.

Summary.

My subject has proved unwieldy; and in merely sketching its outlines I am uneasily aware that I have overstepped the usual bounds of an Address. My conclusions -if the term be applicable to results mainly negative-are as follows:

(1) In the present state of opinion regarding the glacial sequence and its interpretation in North Europe, it is premature to attempt the arrangement of the British drifts on this basis.

(2) No proof of mild interglacial epochs, or even of one such epoch, was discovered during the examination of certain typically glaciated districts in England, Ireland, and the Isle of Man; and the drifts in these areas yielded evidence that from the onset of the land ice to its final disappearance there was a period of continuous glaciation, during which the former sea-basins were never emptied of their ice-sheets.

(3) The "middle glacial" sands and gravels of our islands afford no proof of mild interglacial conditions or of submergence. In most cases, if not in all, they represent the fluvio-glacial material derived from the ice-sheets.

(4) The British evidence for the Interglacial hypothesis, though requiring further consideration in some districts, is nowhere satisfactory. Most of the fossiliferous beds regarded as interglacial contain a fauna and flora compatible with cold conditions of climate; and in the exceptional cases where a warmer climate is indicated, the relation of the deposits to the boulder-clays is open to question.

(5) The British Pliocene and Pleistocene deposits appear to indicate a progressive change from temperate to subarctic conditions, which culminated in the production of great ice-sheets, and then slowly recovered.

(6) During the long period of glaciation the margins of the ice-lobes underwent extensive oscillations, but there is evidence that the different lobes reached their culmination at different times, and not simultaneously. The alternate waxing and waning of the individual ice-sheets may have been due to meteorological causes of local, and not of general influence.

Let me add, in closing, that it would have been a more gratifying task if, instead of probing into these outstanding uncertainties, I had chosen to deal only with the many and great advances that have been made during the last twenty-five years in the domain of British glacial geology. With these advances we have, indeed, reason to be well satisfied. But the necessity for further knowledge is insistent; and it is useless to set about the solution of our intricate problem until we have all the factors at command. Even then-" Grant we have mastered learning's crabbed text, Still there's the comment "-and, as I have tried to show, the comment may raise more difficulties than the text itself.

SECTION D. ZOOLOGY.

OPENING ADDRESS BY J. J. LISTER, M.A., F.R.S.,
PRESIDENT OF THE SECTION.

The Life-History of the Foraminifera.

IN the year 1881 the British Association, having completed the fiftieth year of its existence, met again in the city of York, where its first meeting had been held. By way of marking the completion of its first half-century, and also to do honour to the city which had welcomed its initiatory gathering, it was arranged that the president of each section of the Association should be selected from among the past presidents of the whole. At that time botanists and zoologists were not so far specialised into distinct groups as, for better or worse, they have since become, and were still, at any rate for the purposes of the British Association meetings, able to share their deliberations. Section D included, besides that of zoology and botany, the departments of anthropology and of anatomy and

physiology, though the two latter had each its own vicepresident.

The naturalist who was selected to preside in 1881 over the whole section was the veteran zoologist, Sir Richard Owen. By that time all or nearly all the 434 scientific memoirs which stand to his name in the Royal Society's Catalogue had been written. Those dealing with comparative anatomy and paleontology, and they are by far the greater part, constitute, to quote the words of Huxley. "a splendid record; enough and more than enough to justify the high place in the scientific world which Owen so long occupied. If I mistake not, the historian of comparative anatomy and of palæontology will always assign to Owen a place next to and hardly lower than that of Cuvier, who was practically the creator of those sciences in their modern shape." But Owen's presidential address dealt not with the anatomy or relationships of living or extinct animals, nor with any of those views on "transcendental anatomy" which have met with less acceptance. The subject selected was the great Natural History Museum at South Kensington, to the planning and establishment of which the energy of his later years was largely directed. In considering the previous occupants of the chair which I have the honour to hold at this seventy-sixth meeting, I cannot refrain from expressing my sense of the loss which not only his friends, but zoology at large, have sustained in the death, last Easter, of Prof. Weldon, the Linacre Professor of Comparative Anatomy at Oxford.

Trained in the pathways of morphology under Balfour at Cambridge, Weldon's energies were, in the later years of his life, devoted to the endeavour to obtain determinations, by means of exact measurements, of the degree of variation from the normal type to which given populations are subject, and, so doing, to find an approximately exact measure of the action of natural selection.

This enterprise and the methods to be employed formed the subject of his address to this Section in 1898, at Bristol and in 1901, assisted by the high mathematical ability of Prof. Karl Pearson, and in consultation with Mr. Francis Galton, he issued the first number of Biometrika : a Journal for the Statistical Study of Biological Problems. It can hardly be doubted that these and similar methods, if properly applied, will render important service in the elucidation of the problems in which we are all, botanists and zoologists alike, interested; though I may confess, for my own part, that those who prophesy from the biometric side of the church use a tongue which is to me unfamiliar. and that, to my loss, I often go away unedified.

It may appear presumptuous in one who thus confesses his inability to grapple with the mathematical intricacies involved in the application of this method if he attempts to offer anything in the nature of advice to those who use it. Nevertheless I do venture-it may be in the "insolence of office "-to urge that the old adage should be borne in mind recommending that before beginning culinary operations it is advisable first to catch your hare-in other words, to make sure that the problem you seek to elucidate is sound from the standpoint of biology before bringing a formidable mathematical apparatus into action for its investigation.

Apart, however, from any misgivings on the propriety of the occasions on which this weapon has been used, there can be no question that, properly applied, the biometric method is a potent addition to the biological armoury, and in the victories that it achieves Weldon will be remembered as the leader of those who foresaw its usefulness and forged it.

account,

Not the least memorable of the lessons he has left us is the eager and strenuous manner in which he did the work, in many fields of activity, which his hand found to do. And while we thus deplore his loss on our Own as as biologists and friends, sympathy goes out, I am sure, towards the home where our respectful his endeavours found such skilled and devoted assistance. Two reports of the Evolution Committee of the Royal Society have been published since Mr. Bateson's prestdential address on Mendelism, or, as we are now to say. Genetics, two years ago. The coincidence of our meeting with that of the Hybridisation Conference in London. together, as I understand, with the fall of the pea-harves",

will prevent the attendance at Section D of some of the chief workers, though two papers on these lines have been promised us, and some aspects of the matter will, I believe, receive attention at the joint meeting which we hold with the botanists, in which several of the prominent foreign workers at Genetics are expected to take part.

The subject to which I wish to invite your attention is the life-history of a group of lowly organisms, the Foraminifera, which belong to a division of the animal kingdom standing apart from all others in the simplicity of the organisation of its members, the Protozoa.

For the last seventy or eighty years the attention of zoologists has been increasingly given to the Protozoa, not only from the interest arising from the particular study of its members, but because, forming as they do a group apart from other animals, and from most plants, they afford a point of view from which to judge of the results on fundamental questions of biology obtained in these more highly developed organisms.

The problems of the relations between protoplasm and nucleus, the significance of the karyokinetic figures and of chromosomes, the phenomena of fertilisation and the differentiation of sex, are all seen more clearly in the light of the results obtained from the Protozoa.

Apart from their interest from this wider standpoint the study of the Protozoa has, as I need hardly remind you, received a great impulse of late years from the discovery that, like the bacteria and their allies, the action of which in this respect has been longer recognised, many of them are, when they gain a footing in the body, the cause of disease in man and other animals. An essential step in counteracting their influence is a knowledge of their lifehistory and mode of attack. For the proper estimation and interpretation of the facts in the life-history of one organism it is, of course, necessary to be acquainted with its course in allied forms, and in other divisions of the class to which it belongs.

Whether we approach the matter from the philosophical or utilitarian side an essential step is to obtain as completely as possible the life-histories of species belonging to the main groups of Protozoa, worked out in detail. Certain aspects of the Protozoa, such as the shells of the Foraminifera, have received a great deal of attention, and we have much accumulated knowledge on particular phases of the life-histories of many forms, but of how few groups can it be said that we know the life-history of any one species completely! For the last thirty years students of biology have begun their studies with an examination of Amoeba, yet the life-history of the common forms of amoba, occurring in streams and ditches, still remains, notwithstanding shrewd surmises as to its course-I think Prof. Calkins will permit me to say-unwritten.

When, therefore, the progress of knowledge of a group reaches a stage in which the main outlines, at least, of the life-history begin to stand forth clearly, it appears to be a matter of importance, not only to the students of that particular group, but, as a standard of comparison, to those of allied groups.

Such a stage has recently been attained in the study of the Foraminifera, and we are now able to sketch with some certainty the general course of the life-history. I have thought, therefore, that the occasion may not be inopportune for me to put the ascertained facts before you, and endeavour to set them in the light of our knowledge of other forms of Protozoa.

The zoologist who for the last twelve years has been preeminent in the investigation of the Protozoa was Fritz Schaudinn, whose early death occurred last June. Beginning his work in F. E. Schulze's laboratory at Berlin, his earlier investigations were directed to the Foraminifera, to the knowledge of which he made important contributions; and three years ago he published an account which, as we shall see, completed the main outline of their lifehistory. His short papers on Actinophrys and various forms of Amoeba embody observations of the highest interest. Turning to the investigation of the Sporozoa, he was soon led to devote his attention more especially to the organisms which produce disease, and his latest achievement was to demonstrate the cause of one of the greatest Scourges of humanity.

Much of his work rests on preliminary accounts of investigations which his splendid activity in research left him no time to publish in detail-though we may hope that, in some cases at least, it may be found possible for the fuller accounts to appear. The papers which he did complete, such as those dealing with the Alternation of Generations in Coccidia and in Trichosphærium, are not only contributions of first-class merit, but models of research and exposition. In all his work he maintained the broad zoological point of view, and his results on the Amoeba associated with dysentery are elucidated by those obtained in the study of the Foraminifera. In his insight into the essentials of the problem before him, and his fertility in technical resources, he was, I venture to think, without a rival.

Having chosen so special a subject, I will endeavour first to set forth briefly the elementary facts of the structure of the Foraminifera, in order that those of my audience who are unfamiliar with them may be able to follow.

In the hollows between the ridges on a ripple-marked stretch of sand it may often be noticed that the surface is whiter than elsewhere. On scooping up some of the sand and examining it with a lens it will be found that the whiteness is due in part, no doubt, to fragments of shells of molluscs of one kind or another, but in part to the presence of complete shells of minute size and the most exquisite shapes. Microscopically examined it will be found that in nearly all cases the shells are made up of a number of separate compartments or chambers, communicating with one another by one or more narrow passages, and disposed in some regularly symmetrical plan. In some the arrangement is a flat spiral, like that of a watch spring; in others helicoid, like a snail's shell. In some the series of chambers may form a straight or slightly curved line, or they may alternate on either side of a straight axis. There is great variety in the plans on which the shells may be built. They differ, too, in texture; some are transparent, and their walls are perforated by multitudes of minute pores, setting the interior of the chambers in direct communication with the outer world, while in others the walls are semi-opaque, white, and glazed like porcelain, and such perforations are absent. The shells are composed, for the most part, of carbonate of lime contained in an organic chitinous matrix, but in many cases grains of sand are included in the walls.

The planispiral chambered shells present such a close resemblance to the shell of a Nautilus that for a long time, notwithstanding their diminutive size, many of them were actually included in that genus, among the cephalopod mollusca. As knowledge advanced the Cephalopoda were divided by D'Orbigny into two groups: the Siphonifères, in which, as in Nautilus, the Ammonites and Spirula, the chambers are in connection by a siphon; and the Foraminifères, in which they communicate by pores.

If instead of examining the empty shells left stranded on the shore we take seaweed from shore pools or from shallow water and separate the adherent particles by means of a sieve, similar Foraminiferous shells will be found in the sand which comes through, and these will usually contain the live animal. If glass slides are set in the vessel on the sand, overnight, some of the animals will generally crawl on to them, and they may then be taken out and examined. About these active animals, springing from various points at the periphery of the shell, are multitudes of slender threads, forming fan-like or sheaf-like groups, by which the animal is attached to the substratum, and by which it moves. They are composed of a clear hyaline substance-protoplasm-containing scattered granules. If the animal is killed and the shell dissolved by a weak acid, no organs, such as muscles, stomach, brain, and so forth, are found in the interior, but the same granular protoplasm is found to fill the interior of all the chambers. As in the Protozoa in general, all the elementary functions subserved by the organs of other animals are performed by the undifferentiated protoplasm. It was not until 1835 that the simple character of the

1 Unt. ub. Generationswechsel bei Coccidien. Zool. Jahrbücher. Anat. Bd. 13. 1000

2 Unt ub. Cenerationswechsel von Trichosphaerium, Abh. Akad. Berlin. 1999. Anhang.

soft parts filling the shells of Foraminifera was recognised by Dujardin. He pointed out that, far from being allied to such highly organised beings as the cephalopod mollusca, they belonged to the simplest forms of animal life, such as Amoeba, and proposed the name Rhizopoda, which is still in use, for the class containing them.

For many years, however, the correctness of Dujardin's views was matter of dispute. One of the first zoologists to recognise their truth and confirm them was the distinguished Yorkshire naturalist, Prof. Williamson, who in 1849 published his memoir "On the structure of the shell and soft animal of Polystomella crispa, " in which, for the first time, the internal structure and the relation between the chambers were correctly described.

In the specimens described by Williamson the shell of Polystomella has the following structure. Externally it is a nearly biconvex shell, symmetrical about a median plane, and with a keel-like projection at the margin. In young specimens sharp points like those of a spur often project from the keel. The chambers of which it is composed are arranged in a spiral. They are convex towards the mouth, i.e. on their anterior faces, and concave in the opposite direction. Moreover, each is produced on either side into a process, or alar prolongation, projecting towards the axis about which the spiral turns, i.e. towards the convex prominence at the centre of each face. Thus each chamber of an outer whorl of the spiral is placed, as it were, astride of the next inner whorl, and the last whorl of the spire completely hides all the previously formed chambers from view. Čareful examination of the anterior face of the terminal chamber reveals a row of foramina along the line where the chamber, including its alar prolongations, rests against the whorl which it bestrides. It results from what has been said that they present a V-shaped line. These foramina are the main openings by which the cavity of the last chamber opens to the exterior. Each chamber of which the shell is composed has been in its turn the terminal chamber, and the openings which then led to the exterior subsequently form communications leading from chamber to chamber. As we trace them back to the earlier chambers they become fewer in number until only a single foramen is found between the chambers. In specimens of the type we are considering a comparatively large globular chamber is the starting point from which growth proceeded. A short passage leads to the second chamber, which has a peculiar shape, being applied to the sphere, produced at one end into a point, and abutting at the other against the third chamber. From this onwards the typical shape is gradually assumed, though in these earlier chambers the alar prolongations are absent. A character of this genus is the presence of the line of pocket-like processes along the posterior margin of the chambers. It was not clear, until Williamson's paper was published, that these ended blindly and did not communicate with the chamber behind. The outer walls of the chambers are traversed by multitudes of pores of extreme minuteness, so that the chambers of the outer whorl have this additional means of communication with the exterior. There is, besides the structures described, a system of canals, lying in the thickness of the walls, and communicating with the chambers, but this need not detain us here.

It results from the structure of such a form as Polystomella that in the earliest stage of its existence the whole organism consisted of a single spherical chamber.

It is to be observed that in shells such as Polystomella the shape and mode of growth of the organism at all stages of its development are preserved in the central parts of the shell. These early formed chambers may be, in some types of growth, exposed to observation, or they may be, as in this genus, built in and hidden by the overlapping of the subsequent additions. They may then, however, be examined by making sections of the shell, or in the protoplasmic casts of the interior when the shell is dissolved.

The Foraminifera are found living attached to other objects on the sea bottom from shore pools down to great depths, and from arctic to tropical waters. A small group of them lead a pelagic life suspended in the upper layers of the great oceans, from the surface down, as Dr. Fowler's collections from the Bay of Biscay show, to at 1 Trans. Microscop. Soc., vol. ii. 1849, p. 159.

An attractive feature of their study is the abundance with which they are represented in geological deposits, right back to the Paleozoic period, so that in dealing with them we have that third dimension, the history of the group in the past, wide open to us in which to project our ideas of the course of their evolution.

It was from the study of fossil Foraminifera of the early Tertiary period that the recent advances in our knowledge of their life-history received its impulse.

The later Eocene rocks in many parts of the world abound in discoidal, slightly biconvex Foraminiferous shells, which, from their likeness to coins, have been called Nummulites. The Nummulitic limestones extend across the Old World from the Pyrenees to China, and often attain a thickness of thousands of feet. Visitors to Egypt are familiar with them in the blocks of which the pyramids of Gizeh are built, and the glittering coin-like discs polished by wind and sand and strewn in the desert have attracted notice from remote antiquity.

The structure of a Nummulite is very similar to that of Polystomella, but the most spacious part of each chamber lies in the median plane of the shell, while the alar prolongations are very thin and interrupted by supporting pillars of solid shell substance. Hence the median plane is a plane of weakness, and the shell readily splits into planoconvex halves, the broken surfaces exposing a section in the median plane of all the chambers of which it is built.

1

It has long been recognised that while the great majority of the specimens of Nummulites occurring in a deposit attain a certain moderate size, a few are found scattered through it the diameter of which far exceeds that of the others. On examining median sections of the smaller specimens it is usually found that the spiral series of chamberstarts from a large and nearly spherical chamber, readily visible to the naked eye, and occupying the centre of the shell, while in the large specimens the spiral series is continued to the centre, where in carefully prepared sections it may be seen to take its origin in a spherical chamber of microscopic size.

Although the two forms were thus found to be associated in the same beds, and to agree with one another closely except in the size to which they grow and in the characters of the central chambers, they were given separate specific names, and attention was called to the puzzling occurrence of these associated "pairs of species," a large and a small one, in various deposits.

It was especially by the labours of De Hantken anl De la Harpe that this phenomenon was brought to light. the latter palæontologist formulating his "Law of the association of species in pairs" as follows: "Nummulites appear in couples; each couple is formed of two species cl the same zoological group, and of unequal size. The larg species is without a central chamber, the small always has More than sixteen pairs of species of Nummulites and the allied genus Assilina, associated in this manner have been enumerated.

one.

In the year 1880 Munier-Chalmas brought before the Zoological Society of France his conclusion that the kinds thus associated were not in fact distinct species but two forms of the same species-that, in fact, the species o Nummulites were dimorphic. He also expressed the opinion that the phenomenon of dimorphism would be found to be of general occurrence among the Foraminifera.

To this view, which further investigations have shown to be entirely correct, Munier-Chalmas added a corollary as to the nature of the relation between these two forms. which was wrong. This, however, need not detain us here.2 Whether he was set against Munier-Chalmas's views by the error of part of them, or for whatever reason De la Harpe failed to recognise, before his untimely death 1 Usually, because the young of the other type occurs among the smalle specimens.

2Cp. the article by the author on "Foraminifera" in Lankester S "Treatise on Zoology," Part I., Fasc. 2, p. 47; and "On the Dimorphi-c of the English Species of Nummulites, &c. P.R.S, vol. lxxi. B P. 298.

which occurred shortly after, the truth which they contained.

Following up the clue which had been found, MunierChalmas and his colleague Schlumberger examined the shells of a large series of forms, especially of the Miliolidae. It was shown, in a fine series of papers, that the phenomenon of dimorphism was present here too, and may find its expression, not only in differences in size of shell and of central chamber, but also in the plan in which the chambers of the two forms are arranged.

While they differ conspicuously-though, as we shall see, in very varying degrees-in the sizes of the initial chamber, it is by no means the case that in all species, as in those of the genus Nummulites we have considered, the size actained by the completed test presents so marked a difference. It is, in fact, more usual for the individuals of the two forms of a species to attain approximately the same size on the completion of growth, though standing so contrasted in the size of the initial chambers.

The names megalosphere and microsphere have been given to the large and the small initial chambers, and the two forms are generally known as the megalospheric and microspheric respectively.

The examination of other groups of Foraminifera has abundantly confirmed the view that the phenomenon of dimorphism is widely prevalent among them.

The Life-history of Polystomella crispa. Turning now from the consideration of the shells of Foraminifera to the living animals, let us inquire what ught has been gained from them on the problem of the significance of the phenomenon of dimorphism.

If a large batch of individuals of Polystomella crispa be killed with a reagent which dissolves the shell, though preserving its protoplasmic contents, it will be found, on xamining the casts so obtained, that besides those of the type described and figured by Williamson with a comparatively large initial chamber (about 60 μ), and these are by far the most abundant, there are others in which the initial chamber is much smaller (about 10 μ). In other words, megalospheric and microspheric individuals occur in the batch, as among the fossil shells of Nummulites, preserved in the Eocene strata.

On staining them another point of difference appears. A single large nucleus is found in the majority of the megalospheric forms, while in the microspheric a number of small nuclei lie in the chambers most remote from the mouth of the shell.

hours, the whole of it has come out and lies massed between the shell and the supporting surface and, within the area formerly covered by the halo. The internal protoplasm is darkly-coloured with brown granules, and the whole mass is during this time the seat of involved streaming movements. Clear spots make their appearance, and gradually the protoplasm collects about these and separates into as many spherical masses, which remain connected by a felt of hyaline pseudopodia. Some 200 is a common number to be found. Not long after they have become distinct it may be noticed that each attains a shining coat -the indication that a shell has been formed, a small aperture being left in each for the passage of the pseudopodia. After lying in close contact for some hours, the spheres rapidly and simultaneously draw apart from one another, and within half an hour from the beginning of the movement they are dispersed over a wide area, and each becomes the centre of a system of pseudopodia of

its own.

The whole of the protoplasm of the parent is used up in the formation of the brood of young, the shell being left empty. The process from the first appearance of the halo to the dispersal of the young is complete in about twelve hours.

In a short time the protoplasm which lies outside the aperture of each of the spheres secretes the wall of a second chamber of characteristic shape, and the young individual is then clearly recognisable in size and shape as the twochambered young of the megalospheric form. Each of the spheres was, in fact, a megalosphere. The microspheric parent has given rise to, indeed it has become, a brood of megalospheric young.

Even before the formation of the megalospheres small rounded, faintly staining nuclei can be seen in stained preparations of the emerged protoplasm, and the latter takes a deep flush owing to the presence of minute particles of chromatin. I am not aware that the origin of these nuclei has been directly observed, but it appears highly probable that they arise by the gathering together about new foci of the staining material distributed through the protoplasm of the microspheric parent.

The Megalospheric Form.

When the megalospheres have become formed their protoplasm contains abundance of irregular chromatin masses, which are at first diffused, and obscure the rounded nucleus near the centre, but I am inclined to think that it is the latter which grows into the large nucleus, the

The Microspheric Form.

The microspheric form has many small nuclei, even at an early stage of growth. These nuclei consist of a homogeneous ground substance with many small nucleoli scattered through it. They lie in the chambers near the centre of the shell, and increase in number by simple division. They also exhibit a remarkable phenomenon to which I shall have to recall your attention later. Though several of the nuclei, and especially those that have recently divided, have a rounded contour, many of them are highly irregular in outline, giving off processes which extend in branching irregular strands, staining deeply with nuclear stains, into the protoplasm. Free shreds of such strands lie scattered in the chambers in the neighbourhood of the nuclei, and in large specimens of the microspheric form it is common to find the protoplasm crowded with such deeply staining strands, and with no trace to be found of the rounded nuclei present in the earlier stages. It is difficult to avoid the conclusion that the nuclei, after increasing in number by amitotic division, give off the strands and are ultimately wholly resolved into them.

In a culture of Polystomella it is common to find a mode of reproduction which on examination will be found to be that of the microspheric form. It is best followed when occurring in a specimen attached to a glass slide. In the early phases these specimens are distinguished by a great increase in the number of pseudopodia issuing from the shell, so that the latter appears when seen by transmitted light to be surrounded by a milky halo. The protoplasm gradually emerges from the shell until, after some

the greater part of the life of the megalospheric form.

As growth proceeds and the number of chambers increases the nucleus moves on from chamber to chamber, becoming greatly constricted as it passes through the narrow passages of communication. It grows pari passu with the growth of the protoplasm. Numbers of nucleoli are contained in it, lying in a reticulum, and the nucleoli appear to increase in number and to decrease in size as growth advances. Here, too, as in the microspheric form, the nucleus appears to give off portions of its substance into the protoplasm, the path along which it has travelled, through the earlier chambers, being strewn with deeply staining particles of irregular size. Towards the later stages the nucleus loses its compact shape and staining power, and ultimately disappears, and multitudes of minute stained bodies may then be detected scattered through the protoplasm. These become aggregated as distinct nuclei, the protoplasm gathers about them, and they divide by karyokinesis. Then follows a second karyokinetic division, and, the protoplasm having divided correspondingly, the whole contents of the megalospheric shell emerges as a multitude of minute biflagellate zoospores, some 4 μ in diameter.

It so happened that I had been working at the lifehistory of the Foraminifera at the same time as Schaudinn, though in ignorance of his work. The results that I have 1 F. Schaudinn, "Die Fortpflanzung der Foraminiferen, und eine neue Art der Kernvermehrung," Biol Centralblatt, Bd. xiv. N. 4, February, 1894. Ueb. d. Dimorphismus der Foraminiferen," Sitz. Ber. d. Ges. naturf. Fr. su Berlin, 1895, N. 5.

J. J. Lister, "Contributions to the Life-history of the Foraminifera," Phil. Trans., vol. c xxxvi. B. (1895), p. 401.