into meso- and meta-nephros; the meso-nephros becomes connected with the male generative organs, and loses its excretory function, while the metanephros persists as the functional kidney. I have, however, fully discussed the evolution of the Vertebrate excretory system in my papers already quoted on their development, and need not refer further to it here, except to point out that there is every reason to believe that the nephridia were originally segmental, one for each somite, that this segmental arrangement is, with the specialisation of the kidney, soon lost as it is in other organs.

ON THE STRUCTURES KNOWN AS PRIMITIVE STREAKS.

I may conclude this paper by a short review of these

structures.

(1) They are always connected with the formation of the

mesoblast.

(2) They are never, so far as I know, found in free larvæ. They are confined to the embryonic phase of development, and are only found in animals which undergo a considerable part of their development in the egg; in other words, only in eggs well-stocked with food yolk, or in eggs which have lost the food yolk. On the other hand, a primitive streak is not universally present in such cases, e. g. Cephalopoda, Elasmobranchii, Amphibia, Crustacea.

(3) They are always median and unpaired in their origin, but may in later development become grooved and present traces of a bilateral structure.

(4) They are always caused by rapid proliferation of cells, apparently from the epiblast.

(5) Their position seems to vary in different animals.

In Vertebrata, when present, the primitive streak is placed mainly behind the blastopore (according to Strahl1 not entirely so in Lacerta, but this is not quite clear from his figures).

In Peripatus it is placed behind the blastopore, and, when the blastopore has divided, behind the hinder division (fig. 4).

1 Arch. f. Anat. u. Phys.,' 1882.

In other Arthropoda in which a primitive streak is present, its position with regard to the blastopore cannot be determined; because the blastopore is not present in those cases in which there is a primitive streak.

With regard to the two first cases the blastopore of the Vertebrata closes, and the anus is subsequently (very late) formed within the area of the primitive streak.

In Peripatus, however, the hinder division of the blastopore does not close but travels slowly back over the area occupied by the primitive streak to its position at the hind end of the body.

I may here mention a fact which I observed last summer in the newt (Triton cristatus). In this animal the blastopore appears not to close but to persist as the anus. This statement is based on surface views of a large number of embryos from the stage when the egg is round until hatching. In all these stages I never saw an embryo without an opening at the hind end of its body. I very much regret that I have not had time to confirm this observation by means of sections.

If true it is most interesting as being the only known case in which the blastopore of Vertebrata actually persists as the anus.

In the case of larvæ which leave the egg at an early stage of development, no primitive streak is developed, but the mesoblast partly grows in from the lips of the blastopore, and partly arises as mesenchyme.

In Amphioxus fourteen pairs of somites are derived as hypoblastic pouches, the remainder are formed from hypoblastic tissue, the exact behaviour of which is not explained by Hatschek.

In those Vertebrata with primitive streak, the anterior somites may be regarded as arising from hypoblastic mesoblast; but the greater part are formed from primitive streak mesoblast.

In Peripatus, the mesoblast arises behind the blastopore from the primitive streak, and grows forward as two bands, exactly as in worms; but it arises from a primitive streak.

I do not think any really satisfactory explanation can be offered at present of these facts. I venture, however, to suggest the following as an attempt at an explanation.

In many living Triploblastica the embryo leaves the egg at a very early stage as a larva; at a stage in which it is little more than a gastrula. Inasmuch as the parent of this ancestor has differentiated nephridia and muscles, &c., it is easily conceivable that the larva should precociously acquire as much of these organs as it requires. Hence mesenchyme. This larva is a small animal, and does not require a pouched gut; its hypoblast becomes specialised for digestion; now it would obviously hamper these exceedingly active larvæ if the gut repeated the phylogeny; at any rate, it is easily conceivable that it would be more advantageous if it were possible that the digestive cells should not have to undergo active developmental changes. Hence the mesoblast has to be formed in another way. The methods in which it is formed are, as is well known, various; it nearly always, however, originates at the lips of the blastopore, as the result of the proliferation of a cell, or cells, which do nothing else but divide and give origin to the mesoblastic bands. This, as I have suggested above, may be looked upon as a modified development of that of the ancestral archenteron, which became pouched, and gave rise to somites (secondary invagination).

In those animals in which this larval phase has become merged in the embryonic development, this process is continued; but the area from which the major part of the mesoblast arises, i. e. from which the secondary invagination takes place, is larger. This may obviously be explained as being due to the fact that, the development being protected, it is not important that the amount of growing tissue present at any given moment should be as small as possible, in order not to hamper the larva.

On this view Amphioxus presents a most surprisingly primitive development, so far as its somites are concerned.

I need hardly point out that the prevailing order of develop

D

ment, from before backwards, is just what would, a priori, be expected. The larva, being a free swimming animal, requires sense organs; it therefore develops its anterior part first and the organs belonging to this region of the body.

JOURNAL OF MICROSCOPICAL SCIENCE.

EXPLANATION OF PLATES II & III,

Illustrating Mr. Sedgwick's Paper on the "Origin of Metameric Segmentation."

Complete List of Reference Letters.

A. Anus. a. Anterior end of young embryo. 4. P. Abdominal pore. B. Body-wall. C. H. Cerebral hemisphere. C. Longitudinal canal connecting pouches of archenteron. E. Ectoderm. G. Gill pouch. H. Heart. K. Nephridium. K. D. Longitudinal duct of Nephridia (segmental duct). M. Mouth. M. A. Coalesced part of primitive mouth. ME. Mesenteron. me. Edge of mesenteries. M. S. Mesoblastic Somites. N. Nervous system between mouth and anus. N. Præoral part of nervous system. N2. Postanal part of nervous system. N. C. Neural canal. Ne. Posterior opening of neural canal. O. External openings of Nephridia. P. Pouches of archenteron. P. Præoral lobe. Py. Anterior opening of neural canal. Si. Siphonoglyphe. Si'. Upper end of siphonoglyphe projecting beyond general edges of lips. St. Wall of stomodum.

Figs. 1-5.-Five young embryos of Peripatus capensis, ventral view. From drawings by Miss Balfour. a. Denotes the anterior extremity.

FIG. 1.-Youngest embryo, with slightly elongated blastopore.

FIG. 2.-Embryo, with three somites and elongated blastopore.

FIG. 3.-Embryo, with five somites. The blastopore is closing in its middle portion.

FIG. 4. The blastopore has completely closed in its middle portion and given rise to two openings, the future mouth and anus. The primitive streak is deeply grooved.

FIG. 5.-Embryo, with about thirteen somites; flexure of hind part of body commenced. The remains of the original blastopore are present, as the mouth placed between the second pair of somites, and the anus placed on the concavity of the commencing flexure of the hind part of the body.

FIG. 6.-Stomodæum of Peachia, laid open so as to show the siphonoglyphe. This figure was very kindly drawn for me by Mr. W. F. R. Weldon. T. Tentacles. St. Wall of stomodæum. Si. Siphonoglyphe. Si'. Upper end of siphonoglyphe, projecting beyond the general edges of the lips. B. Bodywall. me. Edge of mesenteries.