STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 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 
lke 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 
enyeloped, is secreted by 
the nidamental gland, an organ largely developed in the female 
gasteropods and cephalopods (Fig. 50, n). 
Development. The molluscan ovwm consists of a coloured 
yolk (vitellus), surrounded by albumen. On one side of the 
yolk is a pellucid spot, termed the germinal vesicle, haying 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 sperma- 
tozoa.t After impregnation, 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 epitheliwm 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 
Be 
NA 
Fig. 29. Spawn of Doris.* 
* Nidamentai ribbon of Doris Johnstont. (Alder and Hancock.) 
{+ No instance of “partheno-genesis”? is known among the mollusca; 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 fwo 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 Lovén (on certain bivalve mollusca), 
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 pygmeum were distinctly seen to penetrate 
in succession the outer envelopes of the ova, and arrive at the vitellns, when the7 dis- 
appeared. With respect to the “germinal vesicle ;” according to Barry, i! first 
