Manchester Memoirs, Vol. I. (1906), No. 3. 71 



mously identified with the primary endoderm — being 

 wholly excluded from it. Watase, indeed, goes so far as 

 to say on the strength of this result that the digestive 

 systems of a squid and a snail are not homologous. 

 Earlier observers, it may be added, had also failed to 

 trace the endoderm into the gut. And this is not a 

 solitary example. In 1884 Witlaczil ^'^ described the 

 origin of the alimentary canal in Aphis from stomodaeum 

 and proctodaeum alone ; Voeltzkow'^ has maintained the 

 same of Musca ; Lecaillon"" of various Coleoptera; while 

 Heymons,^^ it is interesting to note, though denying to 

 the yolk-cells any share in the development of the gut 

 in Orthoptera, has shown that in Lepidura and Cmn- 

 podea the digestive cavity arises in the fashion required 

 by theory from the primary endoderm. 



Physiologically it is not difficult to understand these 

 anomalies ; they find a parallel, for example, amongst the 

 Vertebrates in the behaviour of the yolk-nuclei of Elasmo- 

 branchs and Teleostei, structures set aside for the 

 elaboration of the yolk but playing no part in the 

 production of the embryo. Morphologically, however, 

 they offer obvious and insuperable difficulties to the 

 ordinary theories of gastrulation. 



Nor is it merely in the development of the digestive 

 cavity that this want of harmony between origin and 

 destiny makes itself so painfully felt. Irregularities of 

 this kind are exhibited by all the germ-layers, and often 

 in the most startling and irrefutable manner. For 



'^ Zeitschr. wiss. ZooL, vol. 40, p. 559 — 696, 1884. 



^^ Arb. Zool. Inst. IViirzbnrg, vol. 9, p. i — 43, 1889. 



-" " Rech. sur Toeuf et devel. embiyon. de quelques chrysomelides." 

 Theses Facult. sci. Fat-i?, ser. A, no. 299, 1898 ; abstr., Zool. Centralblatt, 

 vol. S, p. 813 — 6, 1898. 



■^^ Sitzungsb. Gesell. Naturf. Freunde Be}-lin, 1897, p. ill — 123; 

 Sitzungsb. Akad. Wiss. Berlin, 1894, pt. i, p. 23 — 27. 



