32 



FOUNDATIONS OF BIOLOGY 



FIG. 9. Life history of Sphaerella lacustris. a, b, c, d, asexual cycle; a, w,x, y, z, 

 sexual cycle, a, dormant cell enclosed within ' a protective cyst wall which has 

 ruptured to allow the enclosed protoplast to escape; b, division of the protoplast to 

 form four spores (c) each of which grows, develops two flagella, and assumes the typical 

 'adult' free living form of Sphaerella (d). This may divide many times, but each cell 

 eventually assumes the dormant form (a) again. Under other circumstances the proto- 

 plast from the dormant form may divide until 32 or 64 small cells(w) are formed. These 

 make their escape and are gametes (x) since they fuse in pairs (y). The composite cell 

 resulting from fertilization is a zygote (z) which soon forms a cyst wall and assumes the 

 dormant phase. 



