210 CLOSE OF THE PRE-EVOLUTIONARY PERIOD 



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 

 Hornblatf], 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. Ail 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 con- 

 nective 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 pre- 

 eminently the layer that forms muscular, nervous, vascular 

 and connective tissue. In view of its functional significance, 

 it may be called the molory 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 (Sciten- 

 plattcn}. The former exhibit almost as soon as they are 



