In other directions also von Baer's work was confirmed and extended by later observers—those parts of it particularly that had reference to the germ-layer theory, and to the concept of histological differentiation. His germ-layer theory was accepted in its main lines by Rathke, Bischoff and Lereboullet, and applied by them to the multitude of new facts they discovered. Rathke, in particular, was a firm upholder of the doctrine, and made considerable use of it in his writings.[327] Even before the publication of von Baer's book he had interpreted in terms of the germ-layer theory sketched by his friend Pander the splitting of the blastoderm which occurs in the early development of Astacus, whereby there are formed a serous and a mucous layer, one inside the other—like the coats of an onion, to use his own expressive phrase.[328]

An ingenious application of the Pander-Baer theory was made by Huxley, who compared the outer and inner cell-layers which form the groundwork of the Cœlentera with the serous and mucous layers of the vertebrate germ.[329] He laid stress, it is true, rather on the physiological than on the morphological resemblance. "A complete identity of structure," he writes, "connects the 'foundation membranes' of the Medusæ with the corresponding organs in the rest of the series; and it is curious to remark, that throughout, the outer and inner membranes appear to bear the same physiological relation to one another as do the serous and mucous layers of the germ; the outer becoming developed into the muscular system, and giving rise to the organs of offence and defence; the inner, on the other hand, appearing to be more closely subservient to the purposes of nutrition and generation" (p. 24). Von Baer had already hinted at this homology in the second volume of his Entwickelungsgeschichte (1837), where he says with reference to the separation of the blastoderm of the chick into two layers. "Yet originally there are not two distinct or even separable layers, it is rather the two surfaces of the germ which show this differentiation, just as polyps show the same contrast of an external surface and an internal digestive surface. In between the two layers there is in our germ as in the polyp an indifferent mass" (p. 67). The terms ectoderm and entoderm were introduced by Allman[330] in 1853 for the two cell-layers in the Hydrozoa.

Remak is the second great name in the history of the germ-layer theory. 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 evaginations, 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 canal and its derivatives, as the liver, lungs, pancreas, kidneys, thyroid, thymus, etc. These parts are all concerned in the processes of assimilation 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 sexual 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 canal 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).

His Darmfaserplatten form the nervous and muscular tissue of the alimentary canal 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 illustrated in Fig. 12.

Fig. 12.—Transverse Section of Chick Embryo. (After Remak.)

h.Epidermis.hp."Hautplatte"x.Edge of the smniotic fold.
m.Spinal cord.andph.Pleuro-Peritonial cavity.
mu.Dorsal plate.um.
ug.Pronephric duct.mp."Mittelplatte"d.Epithelium of alimentary canal.
pa.Aortic root.df."Darmfaser platte."