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 _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. 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. Two volumes of the _Suites a Buffon_. [304] _U. d. Metamorphose der Ophiuren u. Seeigel._, Berlin, 1848. _U. d. Metamorphose der Holothurien u. Asterien._, Berlin, 1851.