(Osteology and Myology). (_Abh. konigl. Akad. Wiss.
Berlin_, for 1834, pp. 65-340, 9 pls., 1836.) Also separately.
[204] "Ueber die Visceralbogen der Wirbelthiere in Allgemeinen und deren Metamorphosen bei den Vogeln und Saugethiere," Muller's _Archiv_, pp. 120-222, 1837.
[205] _Handbuch d. menschl. Anatomie_, iv., p. 47.
[206] This was shown by Serres (_Ann. Sci. nat._, xi., p.
54 f.n., 1827), who found in a human embryo a long cartilaginous piece extending from the ear-ossicles to the inside of the lower jaw, and suggested that it was the foundation of the permanent mandible.
[207] _Abhandl._, i., p. 102, 1832; ii., p. 25, 1833. (_Blennius_ paper).
[208] _Vergleichende Entwickelungsgeschichte des Kopfes der nackten Amphibien_, Konigsberg, quarto, 276 pp., 1838.
[209] Muller's _Archiv_ for 1838.
[210] _Entwickelungsgeschichte der Natter_, Konigsberg, 1839.
[211] _Bemerkungen uber die Entwickelung des Schadels der Wirbelthiere_, Konigsberg, 1839.
[212] _Handbuch der Physiologie des Menschen_, Koblenz, 1835; Eng. trans. by W. Baly, ii., p. 1615, 1838.
[213] For a full statement of Rathke's conclusions, see the translation given by Huxley in _Lectures on the Elements of Comparative Anatomy_, London, 1864.
[214] _Entwickelungsgeschichte der Wirbelthiere_, p. 142, 1861.
[215] _Embryologie des Salmones_. A separate volume of L.
Agassiz's _Histoire naturelle des Poissons d'Eau douce de l'Europe centrale_, Neuchatel, 1842.
[216] _Untersuchungen uber die Entwickelungsgeschichte der Geburtshelferkrote_, Solothurn, 1842.
[217] Muller's _Archiv_ for 1843, p. ccxlviii.
[218] _Untersuchtingen uber die Entwickelung der Wirbelthiere_, Berlin, 1850-55.
[219] Delivered 17th June 1858. Reprinted in _The Scientific Memoirs of T. H. Huxley_, edited by M. Foster and E. Ray Lankester, vol. i., pp. 538-606 (1898).
[220] _Cf._ Reichert, _supra_, p. 149.
[221] The origin of the pituitary body from the roof of the mouth was first described by Rathke (1839).
[222] _Human Osteogeny explained in two Lectures_, London, 1736.
[223] _De capitis ossei Esocis lucii structura singulari.
Dissert. inaug._ Regiomonti, 1822.
[224] "Ueber das aussere und innere Skelet," Meckel's _Archiv_, pp. 327-76, 1826.
[225] _Vergl. Entwick. d. Kopfes d. nackten Amphibien_ (p.
186).
[226] _Arch. f. mikr. Anat._, xi., Suppl., 1874.
[227] "Om Primordial-Craniet," _Forhandlingar Skand.
Naturf. Mole_, Stockholm, 1842.
[228] Vol. I., General part, pub. 1844.
[229] _Entosphenoid_, Owen.
[230] _Zweiter Bericht zootom. Anstalt zu Wurzburg_, 1849.
[231] _Zeits. f. wiss. Zool._, ii., pp. 281-91.
[232] Muller's _Archiv_ for 1849, pp. 443-515.
[233] _Zeits. f. wiss Zool._, ix., 1858.
[234] _Entw. d. Wirbelthiere_, pp. 139-40, 1861.
[235] _Lectures on the Elements of Comparative Anatomy_.
[236] _On the Archetype of the Vertebrate Skeleton_, p. 5, 1848.
[237] _System der thierischen Morphologie_, Leipzig, 1853.
CHAPTER XI
THE CELL-THEORY.
With the founding of the cell-theory by Schwann in 1839 an important step was taken in the analysis of the degrees of composition of the animal body. Aristotle had distinguished three--the unorganised material, itself compounded of the four primitive elements, earth and water, air and fire, the homogeneous parts or tissues and the heterogeneous parts or organs, and this conception was retained with little change even to the days of Cuvier and von Baer. Those of the old anatomists who speculated on the relations of organic elements to one another were dominated by Aristotle's simple and profound classification, and proposed schemes which differed from his only in detail. Bichat enlarged and deepened the concept of tissue, but the degree of composition below this was for him, as for all anatomists of his time, a fibrous or pulpy "cellulosity," living, indeed, but showing no uniform and elemental structure. It was Schwann's merit to interpose between the tissue and the mere unorganised material a new element of structure, the cell. And, as it happened, a few years before Schwann published his cell-theory, Dujardin hinted at another degree of composition which was later to take its place between the cell and the chemical elements--sarcode or protoplasm.
As is well known, the concept of the cell arose first in botany. Robert Hooke discovered cells in cork and pith in 1667, and his discovery was followed up by Grew and Malpighi in 1671, and by Leeuenhoek in 1695. But they did not conceive the cell as a living, independent, structural unit. They were interested in the physiology of the plant as a whole, how it lived and nourished itself, and they studied cells and sieve-tubes, wood fibres and tracheae with a view rather to finding out their functions and their significance for the life of the plant than to discovering the minutiae of their structure. The same attitude was taken up by the few botanists who in the 18th century paid any heed to the microscopical anatomy of plants. For C. F. Wolff,[238] the formation of cells was a result of the secretion of drops of sap in the fundamental substance of the plant, this substance remaining as cell-walls when cell-formation was completed--no idea here of cells as units of structure.
In the early 19th century, interest in plant anatomy revived somewhat, and much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von Mohl.[239] As a result of their work the fact was established that the tissues of plants are composed of elements which can, with few exceptions, be reduced to one simple fundamental form--the spherical closed cell. Thus the vessels of plants are formed by coalescence of cells, fibres by the elongation of cells and the thickening and toughening of their walls. At this time, interest was concentrated on the cell-wall, to the almost total neglect of the cell-contents; the "matured framework" of plant cells, to use Sach's convenient phrase, was the chief, almost the sole, object of study. And it was natural enough that the mere architecture of the plant should monopolise interest, that the composition of the tissues out of the cells, and the fitting together of the tissues to form the plant should awaken and hold the curiosity of the investigator; even the modifications of the cell-walls themselves, their rings and spiral thickenings and pits, offered a fascinating field of enquiry.
The idea that the cell-contents might show a characteristic and individual structure had hardly dawned upon botanists when Schleiden published his famous paper, _Beitrage zur Phytogenesis_.[240] Schleiden's theme in this paper is the origin and development of the plant cell, a subject then very obscure, in spite of pioneer work by Mirbel. A few years before, Robert Brown had called attention to the presence in the epidermal cells of orchids and other plants of a characteristic spot which he called the areola or nucleus.[241] Schleiden saw the importance of this discovery, confirmed the constant presence of the nucleus in young cells, and held it to be an elementary organ of the cell. He named it the cytoblast because, in his opinion, it formed the cell. It was embedded in a peculiar gummy substance, the cytoblastem, which formed a lining to the cellulose cell-wall. Within the nucleus there was often a small dark spot or sphere--the nucleolus. The nucleus, Schleiden thought, originated as a minute granule in the cytoblastem which gradually increased in size, becoming first a nucleolus (_Kernchen_), and then, by further condensation of matter round it, a nucleus. Several nuclei might be formed in this way in a single cell. New cells took their origin directly from a full-grown nucleus, in a peculiar way which Schleiden describes as follows:--"As soon as the cytoblasts have reached their full size a delicate transparent vesicle arises on their surface; this is the young cell, which at first takes the shape of a very flat segment of a sphere, of which the plane surface is formed by the cytoblast, the convex side by the young cell itself, which lies upon the cytoblast like a watch-glass on a watch" (p. 145). The young cells increase in size and fill up the cavity of the old cell, which is in time resorbed. Cell-development always takes place within existing cells, and either one or many new cells may be formed within the mother-cell. Schleiden's views on cell-formation were drawn from some rather imperfect observations on the embryo-sac and pollen-tube, but he extended his theory to cell-formation in general. Though wrong in almost all respects the theory had at least the merit of fixing attention upon the really important constituents of the cell, the nucleus and the cell-plasma. To Schleiden, too, we owe the conception of the cell as a more or less independent living unity, whose life is not entirely identified with the life of the plant as a whole. "Each cell," he writes, "carries on a double life; one a quite independent and self-contained life, the other a dependent life in so far as the cell has become an integral part of the plant" (p. 138).
So long as the definition of the plant cell embraced little more than the hardened cell-wall it was little wonder that "cells" in this sense were not recognised in animal tissues, except in a few exceptional cases--as in the notochord by Johannes Muller.[242] Careful observation of animal tissues discovered in some cases the existence of discontinuous units of structure, but these were not, as a rule, recognised before 1838 as analogous to plant cells. Von Baer, for example, observed that the young chick embryo was composed partly of an albuminous mass and partly of _Kugelchen_ or little globules suspended in it (_Entwickelungsgeschichte_, i., pp. 19, 144). Since such _Kugelchen_ disposed in a row formed the notochord (i., p. 145) it seems probable that his _Kugelchen_ were really cells. Similarly A. de Quatrefages[243]
in 1834 saw and figured segmentation spheres in the developing egg of _Limnaea_, but he called them globules and did not recognise their analogy with the cells of plants. According to M'Kendrick,[244] Fontana, so far back as 1781,[245] described cells with nuclei in various tissues, and used acids and alkalis to bring out their structure more clearly.
But it was not till 1836-7-8 that a fairly widespread occurrence of cells in animal tissues was recognised. The pioneer in this seems to have been Purkinje, who described cells in the choroidal plexus in 1836,[246] and compared gland cells with the cells of plants in 1837.[247]
Henle in 1837[248] and 1838[249] described various kinds of epithelial tissue, distinguishing them according to the kind of cell composing them; he also discovered the mode of growth of stratified epithelium.
Valentin[250] appears to have seen cells in cartilage and epithelium even before Henle, and to have observed cells in the blastoderm of the chick.
In his report on the progress of anatomy during 1838 Johannes Muller was able to refer to quite a number of papers dealing with the occurrence of cells in animal tissues. In addition to those already noted, he mentions work by Breschet and Gluge on the cells of the umbilical cord, by Dumortier on the cells in the liver of molluscs, by Remak and by Purkinje on nerve cells, by Donne on the cells of the conjuctiva, cornea and lens. He reports, too, that Turpin had compared the epithelial cells of the vagina with the cell-tissue of plants. Muller himself had not only recognised the cellular nature of the notochord, but had observed the cells of the vitreous humour, fat cells and pigment cells, and even the nuclei of cartilage cells. From Schwann (1839) we learn that C. H.
Schults had followed back the corpuscles of the blood to their original state of nucleated cells, and that Werneck had recognised cells in the embryonic lens. A preliminary notice of Schwann's own work appeared in 1838 (Froriep's _Notizen_, No. 91, 1838), the full memoir in 1839, under the title _Mikroskopische Untersuchungen uber die Uebereinstimmung in der Struktur und dem Wachstume der Tiere und Pflanzen_.[251]