THE EGG CELL OR OVUM. 57 



with the middle layer or mesoderm, usually on an area of 

 the epithelium lining the body cavity. In lower animals they 

 often arise somewhat diffusely ; in higher animals their for- 

 mation 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. 41, p. 139). The 

 ovum grows at the expense of adjacent cells, or by absorbing 

 material which is contributed by special yolk glands or sup- 

 plied by the vascular fluid of the body. 



The yolk or nutritive capital may be small in amount, 



FIG. 7. Diagram of ovum, showing diffuse yolk granules. 

 g.v. Germinal vesicle or nucleus ; chr. chromatin elements. 



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, Rep- 

 tiles, and most Fishes. 



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 adja- 

 cent cells (e.g., the follicular envelope) : or (c) formed by 



