(307) O. Hertwig. "Ueber d. Hautskelet d. Fische." _Morphol. Jahrbuch_, Vol. II. 1876. (Siluroiden u. Acipenseridæ.) (308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus)." _Morph. Jahrbuch_, Vol. V. 1879. _Feathers._ (309) Th. Studer. _Die Entwick. d. Federn._ Inaug. Diss. Bern, 1873. (310) Th. Studer. "Beiträge z. Entwick. d. Feder." _Zeit. f. wiss. Zool._, Vol. XXX. 1878. _Sweat-glands._ (311) M. S. Ranvier. "Sur la structure des glandes sudoripares." _Comptes Rendus_, Dec. 29, 1879. _Mammary glands._ (312) C. Creighton. "On the development of the Mamma and the Mammary function." _Jour. of Anat. and Phys._, Vol. XI. 1877. (313) C. Gegenbaur. "Bemerkungen üb. d. Milchdrüsen-Papillen d. Säugethiere." _Jenaische Zeit._, Vol. VII. 1873. (314) M. Huss. "Beitr. z. Entwick. d. Milchdrüsen b. Menschen u. b. Wiederkäuern." _Jenaische Zeit._, Vol. VII. 1873. (315) C. Langer. "Ueber d. Bau u. d. Entwicklung d. Milchdrüsen." _Denk. d. k. Akad. Wiss. Wien_, Vol. III. 1851. [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. [152] Whether there is any part of it in many types not so derived requires further investigation, now that it has been shewn by the Hertwigs that part of the system develops from the endoderm in some Coelenterata. O. Hertwig holds that part of it has a mesoblastic origin in Sagitta, but his observations on this point appear to me very inconclusive. It would be very advantageous to investigate the origin of Auerbach's plexus in Mammalia. Before considering in detail the comparative development of the nervous system, it will be convenient shortly to review the present state of our knowledge on the general process of its evolution. This process may be studied either embryologically, or by a comparison of the various stages in its evolution preserved in living forms. Both the methods have led to important results.