93 



slit running dorso-ventrally. Tliese walls are perfectly 

 white and of a very peculiar cheese-like consistency. 

 This posterior glandular portion of the oviduct, after its 

 abrupt beginning, takes again the anterior direction, and 

 runs along to the right of the rectum, the rectum, in fact, 

 lying upon it (fig. 59). It eventually opens by a small 

 orifice into the pallial cavity. 



The ovary is also tubular in section, the wide tubules 

 of which it is composed being arranged at right angles 

 to the surface of the gonad. In a hand section the 

 parallel arrangement of ovarian tubules may be 

 distinguished quite easily with the naked eye. In 

 transverse section they are five or six-sided (fig. 60), 

 and are composed of a thin wall of germinal epithelium 

 (fig. GO, Ge. ef.) — flattened cells — which here and there 

 gives rise to large egg cells. These contain a conspicuous 

 nucleus with nucleolus. One interesting fact is that the 

 germinal epithelium gives rise to delicate follicles of 

 flattened cells, which surround the developing eggs 

 (fig. 60, Foil. ep.). This is seen again in the 

 Cephalopoda, but does not seem to be the rule in the 

 Lamellibranchiata. It is certainly not so in Pecten or 

 Cyclas, where the stalked egg projects freely into the 

 cavity, surrounded only by the egg membrane. 



EMBRYOLOGY. 



Fertilisation takes place internally, but details of 

 the exact procedure have so far not been observed. The 

 eggs, surrounded by a transparent viscous mass of 

 albumen, are laid in capsules which are deposited in 

 considerable numbers attached to each other, and often 

 fixed to floating objects or to mollusc shells, rocks, 

 Crustacea, etc., on the sea bottom. Whelks' egg-capsules 



