GERM CELLS 'AND THEIR FORMATION 



87 



that they are nearly always larger than ordinary cells (Fig. 42). 

 The size of the ovum is not related to the size of the organism 

 producing it, but is in general related to the amount of food 

 substance stored in it, the actually living protoplasm showing 

 much less variation in amount than the deutoplasm. The 

 smallest eggs are those of the Mammals (Figs. 43, 14, VII), in 

 man only 0.25 mm. (250 micra or 0.01 inch) in diameter, others 

 being still smaller 0.07-0.10 mm. (70-100 micro) in the deer, 



v.ex. 



FIG. 42. A. Section through the egg of the lamprey, Petromyzon fluviatilis. 

 After Herfort. B. Spermatozoon, drawn to scale, d.en., dense endoplasm; 

 t'.ra., inner membrane (? vitelline) ; o.m., outer membrane (? chorion) ; p, granular 

 "polar plasm;" v.en., vacuolated endoplasm; v.ex., vacuolated exoplasm; /, first 

 polar body; //, second polar spindle. 



and only 0.065 mm. (65 micro) in the mouse. The largest eggs 

 are, in volume, the largest known cells; such are the " yolks" of 

 birds' eggs, the largest of which are several inches in diameter 

 and equalled in other groups only by the eggs of one of the 

 sharks (Heterodontus) which are nearly 2 inches (4.0 to 5.0 cm.) 

 in diameter. In a very few cases (some Coelenterates and 

 Porifera, and a few worms) the ova may be capable of loco- 

 motion, performing amoeboid movements (Fig. 44), but in 

 nearly all cases they are quiescent, passive structures although 



