OVA 



329 



Ova. The ovum (Fig. 93), or female sexual cell, is the 

 largest cell to be found in the human body. Its diameter is 

 about M.25 of an inch. Its structure is that of a typical cell. 

 When the ovary is developing a part of its covering epithel- 



k 



m 



2 



Ttl 



\ 



FIG. 92. Spermatozoa. 



i, human ( X 600), the head seen from the side; 2, on edge; k, head; m. 

 middle piece; f, tail; e, terminal filament; 3, from the mouse; 4, bothriocephalus 

 latus; 5, deer; 6, mole; 7, green woodpecker; 8, black swan; 9, from a cross be- 

 tween a goldfinch (m.) and a canary (f.) ; 10, from cobitis. (Landois.") 



ium dips down into the substance of the organ and become? 

 walled off by the union of the surface cells above it. A 

 part of this ball of epithelium becomes the ovum, and a part 

 the Graafian follicle for that ovum. The youngest ova are 

 thus found nearest the surface of the ovary. The cell has an 

 enveloping membrane, the vitelline membrane, a protoplasm, 

 the mtellus, a nucleus, the germinal vesicle, and a nucleolus, 



