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. 66, p. 143). 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 accum- 

 ulated in the centre 

 as in the eggs of 

 Insects and Crust- 

 aceans ; or it may 

 be very copious, 

 dwarfing the form- 

 ative protoplasm, 

 as in the eggs of 

 Birds, Reptiles, and 

 most Fishes (Fig. 



3 2 )- 



Round the egg 



there are often 

 sheaths or envel- 

 opes of various 

 kinds - - (a) made 

 by the ovum itself, 

 and then very deli- 

 cate (e.g. the vitelline membrane) ; (b] formed by adja- 

 cent 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 enclos- 

 ing the egg of a skate, are of course formed after fertilisation. 

 Egg-shells must be distinguished from egg capsules or 



FIG. 27. Diagram of ovum, showing diffuse 

 yolk granules. 



A'"-"'. , Germinal vesicle or nucleus; chr. , chromatin 

 elements. 



