Development 41 



a fine thread and is called the " tail." The object of 

 such an extreme modification of the cell becomes clear 

 when it is observed that its tail or flagelltim is capable of 

 a violent lashing movement that propels it rapidly 

 through the fluid in which it finds itself after being dis- 

 charged from the body of the male. This cell is modified 

 for swimming in order that it may meet the ovum, and 

 its great motility also enables it to penetrate the latter 

 when contact with it has been effected. 



For a time the male cells possess great vitality, though 

 their life is short if they fail to meet the ovum, for they 

 carry no food. Even in higher animals, where, in per- 

 forming their function, they are neither exposed to the 

 air nor to a change of temperature, they will live for a 

 time outside the body, and apparently are not injured by 

 many salts in solution, recover from the effects of nar- 

 cotics, and may even regain their motion after being 

 frozen. 



The necessity of such great vitality is especially clear 

 in the case of marine animals like oysters, clams, starfish, 

 and many others, where there is no union of the sexes, 

 but a mere discharge of sexual cells into the water where, 

 often under adverse conditions, they must find each 

 other. 



The ovum and spermatozoon of the oyster, and also the 

 male cells of the quahaug and scallop, are represented 

 in Figure 10. With slight changes this figure would 

 represent equally well the sexual cells of other bivalves — 

 and, in fact, those of almost any sexual animal. The 

 ovum is seen to be relatively large, with a conspicuous 

 nucleus (;/), and bears minute granules of yolk. The 

 long flagellum or tail of the spermatozoon drives the 

 cell with the nucleus forward. The latter structure has a 



