54 REPRODUCTION AND LIFE-HISTORY OF ANIMALS. 



whether male or female, is that they retain the essential 

 qualities of the fertilised ovum from which the parent 

 animal was developed. 



The ovum has the usual characters of a cell : its sub- 

 stance is traversed by a fine protoplasmic network ; its 

 nucleus or germinal vesicle contains the usual chromatin 

 elements ; it has often a distinct sheath representing a cell- 

 wall. 



In Sponges, the ova are well-nourished cells in the middle 

 stratum of the body ; in Ccelenterates, they seem to arise in 

 connection with either outer or inner layer (ectoderm or 

 endoderm); in all other animals, they arise in connection 

 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 like an amoeba, and that of 

 Hydra retains this character. It grows at tfie expense of 

 adjacent cells, or by absorbing material which is contri- 

 buted 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 Mam- 

 mals, of 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, 

 dwarfing the formative protoplasm, as in the eggs of Birds, 

 Reptiles, and some 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) often formed by 

 adjacent cells (e.g.. the follicular envelope) ; or (c) formed 

 by special glands, or by glandular cells in the walls of the 

 oviducts (e.g., the "shells" of many eggs). The envelope 

 is often firm, witness the chitinous coat round the eggs of 

 many Insects. In these cases there is often a little aperture 

 (micropyle) through which alone the spermatozoon can 

 enter. The hard calcareous shells round the eggs of Birds 

 and Tortoises, or the " horny " mermaid's purse enclosing the 



