aserplatten_ 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.

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 principally 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 Begruendung 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 ueber die Fortschritte der mikroskopischen
Anatomie im jahre 1854._ Mueller'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 classical 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.