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 



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^^v nIS*'v-- -HAwr 



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=2.$& ^&~\ 



\-bl 



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- 



yh.&p 



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 invest- 

 inff the entire 



ovum. 



B 



P 



7Q8 _ Sections through the edge of the blastoderm of Sepia at 

 three successive stages, bl. blastoderm ; yk. yolk : yk. ep. yolk- 

 epithelium. (From Korschelt and Heider, after Vialleton.) 



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



