XII 



PHYLUM MOLLUSCA 



775 



each egg, enclosed in a gelatinous sheath, has a longer or shorter 

 stalk. A chorion or delicate transparent egg-membrane, in which 





f>r 



FIG 707 -Sepia, blastoderm at a late stage of segmentation, bl. blastoderm ; yk. yolk. (From 



Korschelt and Heider, after Vialleton.) 



there is an aperture the micropyle immediately invests the 

 egg itself. In shape the egg is oval or spherical. The greater part 

 of the compara- 

 tively small 

 quantity of pro- 

 toplasm lies as 

 a disc-like ele- 

 vation on the 

 surface of the 

 yolk on the side 

 of the egg at 

 which the mi- 

 cropyle is situ- 

 ated. Contin- 

 uous with this 

 germinal disc is 

 a thin- layer of 

 peripheral pro- 

 toplasm inveS FIQ - (| , _ Soctions t |, ru ,mi, ti,,. ,.,1,,,, ,, t the bla-todenn of Sepia at 

 ll\0 the entire three successive stages. I'/, lila-toflenn ; nk. yolk: >/!:. cp. yolk- 



epithelium. (From Korschelt and Hoid'.v. alter Vialleton.) 

 ovum. 



Segmentation (Figs. 706 and 707) is incomplete, being confined 



^ft r " ssry 



