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).

[Illustration: FIG. 12.--Transverse Section of Chick Embryo. (After
Remak.)]

His _Darmf