STEUCTUEE AXD PHYSIOLOGY OF THE MOLirSCA. 



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Fig. 29, Sprr?rh of Doris.* 



In tough albuminous capsules, each, containing numerous germs ; 

 these are deposited singly, orinroT\\s, or agglutinated in groups, 

 equalling the parent ani- 

 mal in bulk (Fig. 83). The 

 nidamental caj^sules of the 

 cuttle-fish are clustered 

 like grapes, each con- 

 taining but one embryo; 

 those of the calamary are 

 grouped in radiating 

 masses, each elongated 

 capsule containing 30 or 

 40 ova. The material 

 with which the eggs are 

 thus cemented together, or 

 enveloped, is secreted by 

 the nidamental gland, an organ largely developed in the female 

 gasteropods and cephalopods (Fig. 50, n). 



Development. The molluscan ovum consists of a coloured 

 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 sui'face. This germinal vesicle is a 

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

 the essential part of the Q^g, from -which the emhri/o is formed; 

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

 tozoa. ■\ After imi)regnation, the germinal vesicle, which then 

 subsides into the centre of the 3'olk, 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 jDresents 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 ^^cinity of the cilia, and they increase in strength, 

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



* Nidamentai ribbon of Dons Johnstoni. (Alder and Hancock.) 

 t Xo instance of " partheno-genesis " is known among the violbisca; tlie most 

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

 lactea. Mull., from the South of Eiu-ope, after being tico 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 I , 



X According to the observations of Professor Loven (on certain bivalve mollusca), 

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

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

 ovary itself. The spermatozoa of cardium pygmcciim were distinctl}- seen to penstrate 

 in succession the outer envelopes of the ova, and arrive at the ritellns, when thej dis- 

 appeared. With respect to the "germinal vesicle;" according to Barry, il first 



