326 



EVOLUTION AND GENETICS 



a gi'eat gap, for the formation and union of gametes demand a 

 differentiation of reproductive cells which the simpler processes 

 of asexual reproduction do not require. 



In the simplest forms of sexual reproduction all individuals are 

 apparently the same and the gametes which they produce show 



Fig. 187. — Life history of Sphaerella lacuslris. a, b, c, d, asexual cycle; a, w, x, 

 y, z, sexual cycle, a, dormant cell; the protective cyst has ruptured to per- 

 mit the escape of the protoplast; b, division of the protoplast to form four 

 spores (c), each of which grows and takes on the adult form (d). This 

 process may be repeated, but ultimately the cells become dormant again. 

 Under some conditions the protoplast divides as in iv, forming 32 or 64 

 small cells. These are gametes. They escape (x) and fuse in pairs (y) to 

 form a zygote (z) which develops a cyst as in a. (From Woodruff.) 



