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(314) M. Huss. “Beitr. z. Entwick. d. Milchdrüsen b. Menschen u. b. Wiederkäuern.” Jenaische Zeit., Vol. VII. 1873.
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[149] The horny teeth of the Cyclostomata are structures belonging to this group.
[150] For the most important contributions on this subject from which the facts and views here expressed are largely derived, vide O. Hertwig, Nos. [306]-[308].
[151] For a very different view on this subject vide Creighton (No. [312]).
CHAPTER XV.
NERVOUS SYSTEM.
Origin of the Nervous System.
One of the most important recent embryological discoveries is the fact that the central nervous system, in all the Metazoa in which it is fully established, is (with a few doubtful exceptions) derived from the primitive epiblast[152]. As we have already seen that the epiblast represents to a large extent the primitive epidermis, the fact of the nervous system being derived from the epiblast implies that the functions of the central nervous system, which were originally taken by the whole skin, became gradually concentrated in a special part of the skin which was step by step removed from the surface, and has finally become in the higher types a well-defined organ imbedded in the subdermal tissues.