No. I.] GERM-LAYERS IN CLEPSTNE. 177 



cases where separate germ-bands are formed concrescence must 

 be conceded. The formation of the vertebrate embryo may be 

 easily regarded as a modification of the same process, and a 

 more rational view has not yet been propounded, so far as I am 

 able to judge. Concrescence seems, indeed, to be the very 

 essence of Sedgwick's theory (No. 36) of the origin of the 

 mouth and anus, and yet he rejects this view as applied to 

 vertebrates. 



It will be obvious to the reader of the foregoing pages that I 

 regard as untenable those theories according to which the 

 somites of segmented animals are derived from gut-pouches. 

 It is not the archenteron, nor yet the mesenteron, in which me- 

 tamerism first exhibits itself. 



36. Sedgwick, Adam. Origin of Metameric Segmentation. Quart. Journ. 

 Mic. Sc, XXIV. Jan., 1884. 



BIBLIOGRAPHY. 



12. Balfour, F. M. A preliminary Account of the Development of 



Elasmobranch Fishes. .^«arA ^(?«;'m. i)//c. .Sc, XXII., p. 323. 1874. 

 32. Id. — A Treatise on Comparative Embryology, Vol. II. 1884. 

 9. Bergh, R. S. Die Metamorphose von Aulastoma gulo. Arbeiten a 

 d. zool.-zoot. Inst, in WUrzbtcrg:, VII., H. 3, p. 231. 1885. 



13. Id. — Uber die Metamorphose von Nephelis. Zeitschr. f. wiss. Zool., 



XLI., H. 2, p. 284. 1884. 



20. Id. — tJber die Deutung der allgemeinen Anlagen am Ei der Clepsinen 



und der Kieferegel. Zool. Anz., No. 216. 1886. 



21. Id. — Die Entwicklungsgeschichte der Anneliden. Kosmos, II., p. 401. 



1 886. 



29. Id. — Thatsachen aus der Entwicklungsgeschichte der Blutegel. Zool. 



Anz., VII., No. 160, p. 90. Feb. 18, 1884. 



30. Id. — Die Excretionsorgane der Wiirmer. Kosmos, 11., ^. ^'j. 1885. 

 11. BuTSCHLi, O. Entwicklungsgeschichtliche Beitrage. Zeitschr. f. wiss. 



Zool, XXIX., p. 239. 1877. 



34. Fraipont, Julien. Polygordius. Fauna u. Flora des Golfes von 

 Neafel. Monographie XIV. 1887. 



19. FiLLiPPi, F. DE. Sopra I'Anatomia e lo Sviluppo delle Clepsine. 

 Pavia, 1839. 



27. GoETTE, A. Abhandl. z. Entwicklungsgesch. der Thiere, H. 2. 

 Vergleichender Theil. 1884. 

 2. Grube, a. E. Untersuchungen iiber die Entwicklung der Clepsinen. 

 Konigsberg, 1844. 



24. Hatschek, B. Beitrage zur Entwickelungsgeschichte und Morpholo- 

 gic der Anneliden. Sitzb. Akad. Wiss. in Wien, LXXIV. 1876. 



