M. Ussow's Zoohgico-Emhryological Investigations. 217 



layer, and of tlic cylindrical cells of the inward-bent upper 

 germ-hunella, which therefore seems to play the part ot the 

 intestino-glandular layer of the embryos of other animals, 

 representing, I believe, the introverted part of the upper 

 lamella from which the intestinal cavity is formed in Amphi- 

 oxii^*, the simple Ascidia'f, and some Ccelentcrataf, Brachi- 

 opoda§, Vermesll, &c. 



At no single stage of development of the Cephalopod em- 

 bryo is the nutritive vitellus in any way directly connected 

 with the cavity of the intestinal tract, which is completely 

 separated from it, as, indeed, has already been remarked by 

 Kolliker^ and Metschnikofi'**, in opposition to the erroneous 

 statements of older investigatorsff. At the close of the second 

 period the inner nutritive vitellus has the form of a cylinder, 

 from which issue three processes. The inferior, sharp process 

 is, as previously, imbedded in the mantle ; Avhilst the two 

 sickle-shaped lateral processes penetrate into the cephalic lobes 

 behind the eyes, beneath the optic ganglia. The nutritive 

 vitellus passes out of the yelk-sac into the embryo through a 

 cylindrical, gradually narrowing canal formed by the coales- 

 cence of the cephalic lobes ; it is situated between the oesopha- 

 gus, the ganglion pedale^ and the ganglion viscerale. During 

 the whole period of development, the whole mass, both of the 

 inner nutritive vitellus and of the outer nutritive vitellus 

 (whiclris continually passing into the embryo), is gradually 

 absorbed by the cells of different organs and tissues in contact 

 with it. 



This short exposition of tlie results of my tedious investi- 

 gations (upon living embryos and sections of them of different 

 kinds) of the development of the alimentary apparatus of the 

 Cephalopoda contradicts in all points the en-oneous opinion of 

 KoUiker J|, that the intestinal tract originates as a solid cord, 

 in which cavities are only produced subsequently, and con- 

 firms the accurate statements of Metschnikoff§§ as to the pro- 



* Mt5m. de I'Acad. de St. Petersb. tome xi. pi. i. fig. 6, and pi. ii. 

 fig. 20. 



^ Ibid, tome x. pi. i. figs. 10, IG. 



X Gottiug. Nachr. 1808, p. 154 ct seqq. Observations on the Develop- 

 ment of the Coelenterata (in Russian), 1873, pis. ii., iii., iv., vi. 



§ Observations on the Development of the Brachiopoda (in Russian), 

 1874, pi. i. figs. 3, 10. 



II MtSm. de I'Acad. de St. Petersb. tome xvi. pis. i., vi. See also the 

 above-cited memoir of Bobrezky's, pi. i. tigs, 1-8. 



% Loc. cit. p. 86. 



♦* Loc. cit. p. t>4. 



tt Van Beneden, Inc. cit. p. 8 ; Delle Chiaje, Mem. 2nd edit, tome i. 

 p. 40. 



\X Loe. cit. p. 93. §§ Loc. cit. pp. 58, 67. 



