EMBRYOLOGY. 



59 



formation is restricted to distinct regions, and usually to 

 definite organs the ovaries. 



The young ovum is often amoeboid, 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) ; (b) formed by ad- 

 jacent cells (e.g. the follicular envelope) ; or (c) 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, e.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 Amceba 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- 



