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