REPRODUCTION — OVUM. 
377 
transparent and the eggs aggregated and enclosed within a mass of 
albuminous matter. 
The egg originates upon the walls of the lobules composing the 
gonad, and in its early stages resembles an amoeboid cell, but becomes 
rounded and encysted by the reception and storage of nutritive matter, 
through the micropyle, an opening in or prolongation of the vitelline 
Fig. 702. — Ovarian ovum 
of Helix aspersa Miill., 
greatly enlarged (after Hux- 
ley and Martin), showing 
micropyle. 
Fig. 703. — Lobule of the ovotestis of 
Helix hortensis Mull., highly magnified, 
showing the maturation of spermatozoa 
within the lumen of the lobule and of ova 
upon its walls (after Gegenbaur). 
membrane, by which the ovum is fixed to the ovarian walls during 
development, and through which the spermatozoon usually enters for 
fertilization ; this aperture in ova not perfectly mature may be 
stopped by a peculiar lenticular body, known as Keber’s- corpuscle. 
The eggs of the terrestrial species are usually deposited, a few days 
after pairing, in moist and shady places, the animals often forming an 
oblique excavation in the earth for the purpose, in which they partially 
bury themselves during the process, the tentacles being contracted, 
and an interval of four, five or more minutes elapsing between the 
deposition of each egg, the duration of the process occupying from 
twenty to forty hours. 
After deposition the eggs increase rapidly in size, for the mass 
deposited may, owing to this property, in twenty-four to thirty hours 
exceed the total bulk of the animal. 
The eggs of aquatic Gastropods are generally affixed to plants, 
stones or other submerged objects, and Neritina attaches them some- 
times also to the shell itself and that so securely 
that a part of the egg shell may remain per- 
manently fixed to the shell or other surface 
to which it may be applied. 
The ova of some species are placed in 
variously shaped capsules or ovisacs secreted by 
the walls of the oviduct, each capsule containing 
a number of ova, arranged in special modes, peculiar to the different 
‘species ; although in Neritina JlmiatiUs only one egg in a capsule 
becomes developed, the rest discontinuing development at or before the 
Fig. 704. — Egg capsules of 
Valvata cristata (Miiller), 
greatly enlarged (after 
Moquin-Tandon). 
