THE EGG-CELL 



67 



they often arise somewhat diffusely ; in higher animals their 

 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. 89, 2). The ovum 

 grows at the expense of adjacent cells, or by absorbing 

 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 



Fig. 31.— Diagram of ovum, showing diffuse 

 yolk granules. 



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

 elements or chromosomes. 



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. 39). 



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 



