STRUCTURE AND PHYSIOLOGY OF THE IIOLLUSCA. 41 



in tough albuminous capsules, each containing numerous germs ; 

 these are deposited singly, or in rows, or agglutinated in groups, 



equalling the parent ani- 



mal in bulk (Fig. 83). The (^_\=^^< 



nidamental capsules of the 

 cuttle-fish are clustered 

 , like grapes, each con- 

 taining but one embryo; 

 those of the calamary are 

 gi-ouped in radiating 

 masses, each elongated 

 capsule containing 30 or 

 40 ova. The material 

 with which the eggs are 

 thus cemented together, or ^. „„ -^ ^ t^ • ^ 



, . ^ , -, 1 Fig. 29. SpaAVn of Dons* 



L enveloped, is secreted by 



j the nidamental gland, an organ largely developed in the female 



jgasteropods and cephalopods (Fig. 50, n). 



! Development. The molluscan ovum consists of a coloure'd 

 yolk (vitellus), surrounded by albumen. On one side of the 

 yolk is a pellucid spot, termed the germinal vesicle, having a 

 spot or nucleus on its surface. This germinal vesicle is a 



' nucleated cell, capable of producing other cells like itself ; it is 

 the essential part of the e^g, from which the embryo is formed ; 



i but it undergoes no change without the influence of the sperma- 

 tozoa.-\ After impregnation, the germinal vesicle, which then 



I subsides into the centre of the yolk, divides spontaneously into 

 two ; and these again divide and subdivide into smaller and still 

 smaller globules, each with its pellucid centre or nucleus, until 



, the whole presents a uniform granular appearance. The next 

 step is the formation of a ciliated epitlielium on the surface of 

 the embryonic mass ; movements in the albumen become per- 

 ceptible in the vicinity of the cilia, and they increase in strength, 

 until the embryo begins to revolve in the surrounding fluid, t 



* Nidamental ribbon of Doris Johiistoni. (Aider and Hancock.) 

 t No instance of " partheno-genesis " is known among the moUusca ; the most 

 "equivocal" case on record is that related by Mr. Gaskoin. A specimen of helix 

 lactea. Mull., from the South of Europe, after being two years in his cabinet, was dis- 

 covered to be still living ; and on being removed to a plant-case it revived, and six 

 weeks afterwards had produced twenty young ones ! 



% According to the observations of Professor Loven (on certain bivalve moUusca), 

 the ova are excluded immediately after the inhalation of the spermatozoa, and 

 apparently from their influence ; but impregnation does not take place within the 

 ovary itself. The spermatozoa of cardium p7jrjmceum were distinctly seen to penetrate 

 in succession the outer envelopes of the ova, and an-ive at the vitellus, when they dis- 

 appeared. With respect to the " genn;nal vesicle ; " according to Barrj', il first 



