He had the great advantage over von Baer of being able to make use of the cell-theory in interpreting the formation of the germ-layers.

Microscopical technique also had been greatly improved since 1828.[331]

Remak's greatest service was that he put the germ-layer theory in direct relation with the cell-theory by demonstrating the cellular continuity from egg-cell to tissue, and by showing that each germ-layer possessed distinctive histological characteristics. Hardly less important was his clear marking-off of the "middle layer" as a separate and distinct layer of the germ. He it was who introduced the modern conception of the mesoderm, and cleared up the confusion in which Pander and von Baer had left the organs formed between the serous and the mucous layer. Remak's middle layer was a different thing from Pander's ill-defined "vessel-layer"; it included and unified from a new point of view the "vessel" and "muscle" layers of von Baer.

There are in the unincubated blastoderm of the chick, according to Remak,[332] two cell-layers, of which the undermost subsequently splits into two. Three layers are thus formed--the upper, middle and lower. The upper layer differentiates into a medullary plate and an epidermic plate (Remak's _Hornblatt_), and gives origin to the medullary tube with all its ev.a.g.i.n.ations, and to the skin with all its derivatives and pockets.

It forms such diverse structures as the brain, the spinal cord, the eye, the ear, the mouth, hairs, feathers, nails, sweat-glands, lacrymal glands, and so forth. All these parts are connected directly or indirectly with sensation, and the upper germ-layer may accordingly be called the _sensory_ layer. The lower layer gives rise to the epithelium and the proper tissue of the alimentary ca.n.a.l and its derivatives, as the liver, lungs, pancreas, kidneys, thyroid, thymus, etc. These parts are all concerned in the processes of a.s.similation and dissimilation, and the lower layer may accordingly be called the _trophic_ layer. Now between the upper or sensory layer and the lower or trophic layer there exists, in spite of their very different functions, a close histological likeness, for both are essentially epithelial layers. The resemblance is particularly strong if we compare the lower layer with the _Hornblatt_ of the upper layer--both consist of epithelial tissue, and of its derivative, glandular tissue, and form neither vessels nor nerves. The middle layer, on the contrary, forms nerves and muscles, vessels and connective tissue, and little or no epithelium. It does not form all the blood-vessels without exception (and so cannot be called the vessel-layer), for the blood-vessels of the central nervous system are in all probability formed from the upper layer. So, too, it does not form all the nerves and muscles--the optic and auditory nerves and the nerves and muscles of the iris probably arise in the upper layer. But, in spite of these exceptions, its general histological character is so well defined that it may be contrasted with the other two as preeminently the layer that forms muscular, nervous, vascular and connective tissue. In view of its functional significance, it may be called the _motory_ layer, or better, since it forms also the s.e.xual glands, the _motor-germinative_ layer. The middle layer, early in its history, shows a division into dorsal plates (_Urwirbelplatten_) and ventral plates (_Seitenplatten_). The former exhibit almost as soon as they are formed the characteristic proto-vertebral segmentation, the latter split to form the pleuro-peritoneal or body-cavity. Remak describes the latter process as follows:--"In the region of the trunk, where a greater independence of the fate of the alimentary ca.n.a.l and its annexes becomes necessary for the voluntary executive organs, the ventral plates undergo a process of splitting, leading to the formation of the sensitive part of the integument (the _Hautplatten_), the muscular part of the alimentary tube (the _Darmfaserplatten_), and the mother-tissue of the generative organs (the _Mittelplatten_). From the _Hautplatten_ there develops, without the dorsal plates seeming to take any part in the process, the rudiment of the extremities" (p. 79).

[Ill.u.s.tration: FIG. 12.--Transverse Section of Chick Embryo. (After Remak.)]

His _Darmfaserplatten_ form the nervous and muscular tissue of the alimentary ca.n.a.l and its dependencies, and also the heart; the _Hautplatten_ form the general body-wall (exclusive of the skin) and the appendages. In the embryo they line the amniotic cavity. The skeleton and peripheral nerves originate wholly within the middle layer.

Remak's conception of the relations of the three germ-layers to one another and to the body-cavity is well ill.u.s.trated in Fig. 12.

In his germ-layer theory Remak's standpoint is histological rather than morphological. The distinction which he draws between the sensory and trophic layers on the one hand, and the motor-germinative layer on the other, is entirely a histological one. The greater part of his book, indeed, is devoted to a study of the histogenesis of the different organs of the body; he is bent chiefly upon unravelling the part which each germ-layer takes in the formation of each tissue and organ.

His generalisation that two of the germ-layers give rise exclusively or almost exclusively to one kind of tissue excited great interest at the time, and gave the direction to histogenetic research for quite a number of years, though in the end it turned out to be insufficiently founded.

Though Remak's germ-layer theory had thus princ.i.p.ally a histological orientation, it laid down the main lines of the modern morphological treatment of the germ-layers.

[293] _Embryologie des Salmones_, 1842.

[294] _Die Cellularpathologie in ihrer Begrundung auf physiologische und pathologische Gewebelehre_, Berlin, 2nd ed. 1859; Eng. trans., by Chance, 1860.

[295] _Arch. path. Anat. Phys_., vii., pp. 1-39 (1854).

[296] _Bericht uber die Fortschritte der mikroskopischen Anatomie im jahre 1854._ Muller's _Archiv_, 1855. See also 1856.

[297] _Hndb. d. Physiol._, i., 1835.

[298] See Leuckart's reply to Ludwig's criticism, in _Zeit. f. wiss. Zool._, ii., p. 271, 1850.

[299] Leipzig, 1853.

[300] _Souvenirs d'un Naturaliste_, 2 vols., Paris, 1854.

Eng. Trans. as _Rambles of a Naturalist on the Coasts of France, Spain, and Italy_, 2 vols., 1857.

[301] Milne-Edwards later published a cla.s.sical textbook on comparative anatomy and physiology--_Lecons sur la Physiologie et l'Anatomie comparees_, 14 vols., Paris, 1857-80.

[302] Paris, 1834-40. Three volumes of the _Suites a Buffon_.

[303] Paris, 1865. Two volumes of the _Suites a Buffon_.

[304] _U. d. Metamorphose der Ophiuren u. Seeigel._, Berlin, 1848. _U. d. Metamorphose der Holothurien u.

Asterien._, Berlin, 1851.

[305] As I have been unable to obtain a copy of the _Introduction_, the pa.s.sages which follow are taken from the _Rapport_ of 1867, where Milne-Edwards gives a complete exposition of his doctrine, sometimes in the words of the original.

[306] This principle was first developed by Milne-Edwards in 1827, in the _Dictionnaire cla.s.sique d'Hist.

naturelle_. It was probably suggested to him by his studies on the Crustacea, among which the principle is so beautifully exemplified in the concentration and specialisation of the appendages and the ganglionic chain.

[307] Studied by Isidore Geoffroy St Hilaire in his paper _Cla.s.sification parallelique des Mammiferes, C. R. Acad.

Sci._, xx., 1845. Remarked upon by Cuvier, _Regne animal_., i., p. 171, 1817, also by de Blainville.

[308] Cuvier et Valenciennes, _Hist. nat. des Poissons_, i., p. 550, 1828.

[309] _Myxinoiden_, Th. I. _Abh. k. Akad. Wiss. Berlin_ for 1834, pp. 100, 110, 179, etc.

[310] _Vergl. Entw. Kopf. nackt. Amphibien_, p. 101, 1838.

[311] I have not seen the companion volume on palaeontological progression, _Unters. u. d.

Entwickelungsgesetze der organischen Welt wahrend der Bildungszeit unserer Erdoberflache_, Stuttgart, 1858.

[312] "Strobiloid" because of its spiral development. The theory of the spiral growth of plants played an important part in botanical morphology about this time.

[313] _Cf._ Meckel's Principle of progressive Evolution, _supra_, p. 93.

[314] _System der thierischen Morphologie_, pp. 33, 457.

Also C. Bruch, _Die Wirbeltheorie des Schadels, am Skelette des Lachses gepruft_, Frankfort-on-Main, 1862.

[315] In France the vertebral theory was advocated by Lavocat in his _Nouvelle Osteologie comparee de la tete des animaux domestiques_, Toulouse, 1864. It seems also that Lacaze-Duthiers held fast to it even in 1872--_Arch. zool. exp. gen._, i., p. 51, 1872.

[316] _An Essay on Cla.s.sification_, Boston, 1857, London, 1859. He considered the cla.s.sificatory categories to be the categories of the Creator's thought, and hence natural, and in no sense mere conventions.

[317] "Principes d'Embryogenie, de Zoogenie et de Teratogenie," _Mem. Acad. Sci._, xxv., pp. 1-943, pls.

xxv., 1860.

[318] "On the Morphology of the Cephalous Mollusca,"

_Phil. Trans._, 1853, _Sci. Memoirs_, i., pp. 152-92.

[319] "Observations sur les changements de forme que les divers Crustaces eprouvent," _Ann. Sci. nat._ (1) x.x.x., p. 360, 1833.

[320] "Considerations sur quelques principes relatifs a la cla.s.sification naturelle des animaux," _Ann. Sci. nat._ (3) i., p. 65, 1844.

[321] _Supra_, pp. 79-83. Also _Precis d'anatomie transcendante, principes d'organogenie_, Paris, 1842.

[322] The inversion of the organs shown by Vertebrates as compared with Invertebrates is due to the reversed position of the embryo relatively to the yolk! (pp.

821-6).

[323] It is worth while recording that Serres enunciated a "law of symmetry" according to which the embryo is formed by the union of its two symmetrical halves--a law which recalls the "concrescence theory" of His and some modern embryologists.

[324] "Embryologie comparee du Brochet, de la Perche, et de l'Ecrevisse," _Ann. Sci. nat._ (4), i., p. 237, 1854; ii., p. 39, 1854. _Mem. Savans etrangers_, xvii.

[325] _Ann. Sci. nat._ (4) xvi., p. 113, 1861; xvii., p.

88, 1862; xviii., p. 5, 1862; xix., p. 5, 1863.

[326] xx., p. 5, 1863.

[327] Particularly in his _Blennius_ (1833) and _Natter_ (1839).