STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 51 



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 cepha- 

 lopods (fig. 43, 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 surface. This germinal vesicle is a nucleated cell, 

 capable of producing other cells like itself; it is the essential part 

 of the egg, from which the embryo is formed ; but it undergoes 

 no change without the influence of the spermatozoa.* After im- 

 pregnation, the germinal vesicle, which then 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 epithelium on the surface of the embryonic mass ; 

 movements in the albumen become perceptible in the vicinity of 

 the cilia, and they increase in strength, until the embryo begins 

 to revolve in the surrounding fluid. f 



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

 most " equivocal " case on record is that related by Mr. Gaskoin. A speci- 

 men of helix lactea, Mull., from the South of Europe, after being two years 

 in his cabinet, was discovered to be still living ; and on being removed to a 

 plant-case it revived, and six weeks afterwards had produced twenty young 

 ones! 



f According to the observations of Professor Loven (on certain bivalve 

 mollusca), the ova are excluded immediately after the inhalation of the sper- 

 matozoa, and apparently from their influence; but impregnation does not 

 take place within the ovary itself. The spermatozoa of cardium pygmaum 

 were distinctly seen to penetrate, in succession, the outer envelopes of the ova, 

 and arrive at the vitellus, when they disappeared. With respect to the " ger- 

 minal vesicle ;" according to Barry, it first approaches the inner surface of 

 the vitelline membrane, in order to receive the influence of the spermatozoa ; 

 it then retires to the centre of the yolk, and undergoes a series of sponta- 

 neous subdivisions. In M. Loven's account, it is said to "burst" and par- 



D 2 



f 



