GERM CELLS AND THEIR FORMATION 101 



approximately eight hundred and fifty million sperm for each 

 one of the four hundred ova matured during the reproductive 

 period of the female. The volume of the human sperm is 

 roughly only about 1/195000 that of the egg (egg =0.25 mm. in 

 diameter). The sperm of the sea-urchin contains only about 

 1 -400000 to 1/500000 the material in the egg (Wilson). 



In several forms, both Vertebrate and Invertebrate, atypical 

 "giant" spermatozoa are occasionally found (Fig. 52, E). In 

 most instances these are abnormalities resulting from some 

 deviation from the usual course of events during sperm for- 

 mation. But in a few instances (Euschistus) a dimorphism of 

 the sperm seems entirely normal (Montgomery). In such cases 

 the larger sperm heads contain the normal amount of chro- 

 matin, but an excessive amount of linin and karyolymph. 



In marked contrast to the egg, the sperm cells are in very 

 active movement on account of the rapid vibration of the tail. 

 They are always contained within a fluid medium, the seminal 

 fluid, which is either the fluid of the cavities of the body, or a 

 special secretion of certain glands in connection with the 

 reproductive system. In the latter case this fluid is equivalent 

 to a tertiary egg envelope, and like this is sometimes of nutritive 

 value to the germ cells. 



The form differences between ova and sperm give a nice 

 illustration of the modification of structure accompanying a 

 physiological division of labor, which is so well marked here. 

 Both cells contain equal amounts of nuclear substance, but the 

 ovum possesses in addition a large amount of cytoplasm, and 

 often a much larger amount of food substance, while the sperm 

 contains an amount of cytoplasm which represents practically 

 an irreducible minimum, and no deutoplasmic material what- 

 ever. The egg, therefore, provides practically the whole of 

 the extranuclear substance of the developing organism. At 

 the same time the ovum is a passive, non-motile structure. 

 The spermatozoa, on the other hand, are not only extremely 

 motile, but they are produced in very large numbers, conditions 

 correlated with their function of finding the inactive ova and of 

 ensuring the initial stimulus to activity (development) of the 



