328 



REPRODUCTION 



remarkable for their power of locomotion, which is effected by 

 lashings and rotary movements of the tail. 



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

 cell to be found in the human body. Its diameter is about T ^-5- of 

 an inch. Its structure is that of a typical cell. When the ovary 



d 



k 



m 



FIG. 92. Spermatozoa. 



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

 piece;/, 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 between a gold- 

 finch (m) and a canary (f.) ; 10, from cobitis. (Landois.) 



is developing a part of its covering epithelium dips down into the 

 substance of the organ and becomes walled off by 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 mtelline membrane, a 



