EMBRYOLOGY. 59 
formation is restricted to distinct regions, and usually to 
definite organs—the ovaries. 
The young ovum is often amceboid, and that of Hydra 
retains this character for some time (Fig. 70, p. 148). The 
ovum grows at the expense of adjacent cells, or by absorb- 
ing material which is contributed by special yolk glands or 
supplied by the vascular fluid of the body. 
The yolk or nutritive capital may be small in amount, 
and distributed uniformly in the cell, as in the ova of 
Mammals, earthworm, starfish, and sponge; or it may be 
more abundant, sinking towards one pole as in the egg of 
the frog, or accumulated in the centre as in the eggs of 
Insects and Crustaceans; or it may be very copious, dwarf- 
ing the formative protoplasm, as in the eggs of Birds, 
Reptiles, and most Fishes (Fig. 31). 
Round the egg there are often sheaths or envelopes of 
various kinds—(a) made by the ovum itself, and then very 
delicate (e.g. the vitelline membrane); (4) formed by ad- 
jacent cells (e.g. the follicular envelope) ; or (¢) formed by 
special glands or glandular cells in the walls of the oviducts 
(e.g. the “shells” of many eggs). The envelope is often 
firm, as in the chitinous coat around the eggs of many 
Insects, and in these cases we find a minute aperture 
(micropyle) or several of them through which’ the sperma- 
tozoon can enter. The hard calcareous shells round the 
eggs of Birds and Tortoises, or the mermaid’s purse en- 
closing the egg of a skate, are of course formed after 
fertilisation. Egg-shells must be distinguished from egg 
capsules or cocoons, ¢.g. of the earthworm, in which several 
eggs are wrapped up together. 
Male cell or spermatozoon.—This is a much smaller 
and usually a much more active cell than the ovum. 
In its minute size, locomotor energy, and persistent 
vitality, it resembles a flagellate Monad, while the ovum is 
comparable to an Amoeba or to one of the more encysted 
Protozoa. 
A spermatozoon has usually three distinct parts: the 
essential ‘“‘head,” consisting mainly of nucleus, and the 
mobile “tail,” which is often fibrillated, and a small middle 
portion between head and tail, which is said to be the 
bearer of the centrosome. The spermatozoa of Thread- 
