Clopton Havers, in his treatise on the skeleton, described1 the vesicular structure of the marrow, and compared it, when seen under the microscope, to a heap of pearls.

Alex. Monro, primus, in his work on the bones, when writing on the medullary structure, stated that it is subdivided "into communicating vesicular Cells, in which the Marrow is contained. Hence it is that the Marrow, when hardned and viewed with a Microscope, appears like a Cluster of small Pearls. This Texture is much the same as what obtains in the other cellular parts of the Body where Fat is collected, only that the Cells containing the Marrow are smaller than those of the Tunica adiposa or cellulosa elsewhere."

Caspar F. Wolff also recognised that fat was contained in small vesicles, surrounded by a fine membrane. He conceived also that the developing organs, both of plants and animals, consisted of a viscous substance which contained cavities, cells, or bladders which communicated with each other.

Fontana figured the fat vesicles, both free and surrounded by the fibres of the areolar tissue.4

Mirbel, in his botanical writings, published at the beginning of the present century, stated that vegetables were composed largely of cells. He described le tissu cellulaire as composed of les cellules, which were contiguous with each other, so that the walls were in common. These walls were extremely thin and translucent, and sometimes riddled with pores. The term cells was also used both by his contemporaries and successors in their writings on the anatomy of plants.

But anatomists experienced much greater difficulty in distinguishing the presence of cells in the textures of animals. It is true that from the time of Malpighi and Leeuwenhoek, the globules or particles had been recognised in the blood, but it is

1 Osteologia nova, 1691, p. 167.

2 Anatomy of the Humane Bones, Edinburgh, 1st ed., 1726; 2nd ed., 1732. 3 Theoria Generationis, editio nova, 1774; Commentary "Ueber die Nutritionskraft," by Blumenbach and Born, St Petersburgh, 1789.

See his Essay "Sur la structure primitive du corps animal" in his “Traité sur le vénin de la Vipere," Florence, 1781 (Ph. viii. figs. 19, 20).

5 Traité d'Anatomie et de Physiologie végétales, t. i., Paris, An x.; Exposition de la Theorie de l'organisation végétale, Paris, 1809. Ch. Robin, in the article "Cellule," Dict. Encyclop. des Sciences médicales, Paris, 1873, credits Mirbel with having introduced the term "cellules," but the extracts given in the text show that its English equivalent, cells, had been in use for upwards of a century before Mirbel wrote.

only within a comparatively recent period that their cellular structure was determined. Both Bichat and Béclard, in their important treatises on General Anatomy, made no reference to cells as elements of the tissues. Both these authors had chapters du tissu cellulaire or du système cellulaire, a term which had been in use from the early part of the last century. But by the tela cellulosa or cellular tissue, anatomists meant that form of tissue which we now more appropriately call areolar tissue; the so-called cells of which are not microscopic closed vesicles, but areola or spaces bounded by the fibres or lamina of which the tissue is chiefly composed. Béclard, in his description of the adipose tissue, stated that the lobules of fat consisted of microscopic vesicles 16 to bo of an inch in diameter. The vesicles, he says, have walls, but they are so thin as to be indistinguishable. The presence of organised vesicles or globules in the tissues of animals had thus been recognised, but it needed further observations and facts in order to bring them into association with the cells of vegetable tissue.

This was supplied by the discovery in 1831 by the great English botanist, Robert Brown, of the "nucleus" or "areola" in the cells of the epidermis, and other tissues in Orchidea and many other families of plants. Following closely upon this discovery were the observations of Schleiden, published in 1838,5 that the nucleus was a universal elementary organ in vegetables. Schleiden also came to the conclusion that the nucleus must hold some close relation to the development of the cell itself, and he consequently called the nucleus a "cytoblast." Schleiden further discovered that the cytoblasts contained one or more minute circumscribed "spots," or "rings," or "points," which he

1 Anatomie générale, Paris, 1812.

Élémens d'Anatomie générale, Paris, 1823.

3 The term cellular tissue was originally applied to this texture from a fancied resemblance to the proper cell tissue of plants; the walls of the cells of which were believed to be formed of a framework of fine fibres.

"Organs and Mode of Fecundation in Orchidea and Asclepiadeæ," Trans. Linn. Soc., vol. xvi., 1833; reprinted in Miscellaneous Botanical Works, vol. i. p. 511, Ray Society edition.

566

Beiträge zur Phytogenesis," Müller's Archiv, 1838, p. 137.

6 Fontana (op. cit.) figured the “globules” or scales of the epidermis, in which he recognised the nucleus, but he neither gave it a special name, nor knew its importance (plate i. figs. 8, 9, 10).

considered to be formed earlier than the cytoblasts, and which were regarded by him as hollow globules, and were subsequently named by Schwann "nucleoli."

The cellular structure of some of the animal tissues had also begun to be recognised. Turpin had noticed the resemblance between the epithelium corpuscles found in vaginal discharges and the cells of plants. Johannes Müller had discovered that the chorda dorsalis of fishes was composed of separate cells provided with distinct walls, though he did not detect a nucleus in them. Purkinje, Von Baer, Rudolph Wagner, Coste, and Wharton Jones had seen the germinal vesicle within the animal ovum. E. H. Schultz had observed the nucleus in the blood globules, and Valentin and Henle had seen it in the cells of the epidermis. The way was thus prepared for a fuller recognition of the essential correspondence between the elementary tissues of plants and animals and for a wider generalisation. Science had not long to wait for an observer who could take a comprehensive grasp of the whole subject; and in 1839 Theodore Schwann published1 his famous researches into the structure of animals and plants, in which he announced the important generalisation that the tissues of the animal body are composed of cells, or of materials derived from cells:

"That there is one universal principle of development for the elementary part of organisms, however different, and that this principle is the formation of cells."

Both Schleiden and Schwann entertained the idea, which had long before been present in the mind of Grew, that a cell was a microscopic bladder or vesicle. In its typical shape they regarded it as globular or ovoid, though capable of undergoing many changes of form. This vesicle possessed a cell-membrane or wall, which enclosed contents that were either fluid or somewhat more consistent. Either attached to the wall or embedded in it was the nucleus, which in its turn contained the nucleolus. Schwann, however, recognised that many cells did not exhibit any appearance of a cell-membrane, but seemed to be solid, and had their external layer somewhat more compact. As showing, 1 "Mikroskopische Untersuchungen,"1839; and Preliminary Notices in Froriep's Notizen, 1838.

P. 176 of Sydenham Society's translation of Schwann's Memoir.

however, the importance which Schwann attached to the cellwall, I should state that he regarded the chemical changes or metabolic phenomena as he termed them, as being chiefly produced by the cell-membrane, though the nucleus might participate. He explained the distinction between the character of the cell contents and the cytoblastema external to the cell, to the power exercised by the cell-membrane of chemically altering the substances, which it is either in contact with or has imbibed, and also of separating them so that certain substances appear on the inner and others on the outer surface of that membrane. In this way, he accounted for the secretion of urea by the cells lining the uriniferous tubes, and for the changes which not unfrequently take place in the cell-membrane itself by thickening or deposition of layers on or within it.

Schwann described the nucleus as either solid or hollow and vesicular, in the latter case being surrounded by a smooth structureless membrane; whilst the contents of the nucleus, other than the nucleoli, were in his view either pellucid or very minutely granulous.

Both Schleiden and Schwann conceived that in the formation of a nucleus a nucleolus was first produced, that around it new molecules were deposited for a certain distance, and then a nucleus was formed. When the nucleus had reached a certain stage of development, new molecules were deposited upon its exterior so as to form a stratum, which when thin was developed into a cell-membrane, but when thick only its outer portion became consolidated into a cell-membrane. Immediately the membrane became consolidated its expansion proceeded by the progressive reception of new molecules; the cell-wall separated from the cell nucleus, and a vesicle was formed; the intermediate space at the same time became filled with fluid, which constituted the cell contents.

Schleiden contented himself with little more than a simple statement of what he conceived to be the process of cell formation in plants; but Schwann entered into an elaborate survey of cell-life both in animals and plants, and founded on it a theory of cells applicable to all organisms.

Schwann conceived that there existed in organised bodies a solid amorphous or fluid substance to which he gave the name

cytoblastema; this substance might be contained either within cells already existing, or else be situated in the interspaces between cells; and he believed that the cytoblastema for the lymph and blood corpuscles is the fluid lymph-plasma and liquor sanguinis in which these corpuscles float. He held that in the cytoblastema new cells are formed in the manner just described. In animals he says it is rare for cells to arise within preexisting cells; more usually they arise in a cytoblastema external to the cells already present. Schleiden, on the other hand, maintained that in plants new cells were never formed in the intercellular substance, but only within pre-existing cells. The idea obviously present in the mind of Schwann was that the process of cell formation in a cytoblastema had some affinity with that of crystallisation. He figuratively compares the cytoblastema to a mother-liquid in which crystals are formed. He speaks of molecules being deposited around a nucleolus to form a nucleus; of a nucleus growing by a continuous deposition of new molecules between those already existing; and of the cell being formed around the nucleus by a progressive deposition of new molecules; and in more than one passage he indicated that this deposition is a precipitation. He obviously considered the principle of formation of the cell around the nucleus as the same as that of the nucleus around the nucleolus, a process which Valentin subsequently described as heterogeneous circum-position.

But Schwann at the same time showed that, with reference to the plastic phenomena, cells differed from crystals in form, structure, and mode of growth; for whilst a crystal increases only by the external apposition of new particles, a cell grows both by that method and by the intussusception of new matter between the particles already deposited. The difference, he says, is yet more marked in the metabolic phenomena, which he conceived to be quite peculiar to cells. Cells and crystals, however, he considered resembled each other in this point, that solid bodies of a definite and regular shape are formed in a fluid at the expense of a substance held in solution by that fluid, for both attract the substance dissolved in the fluid. Schwann concluded his memoir by advancing, as a possible hypothesis, the view that organisms are nothing but the form under which