148 GENERAL BIOLOGY 



standpoint of reproduction. In one kind, the cells 

 divide into microgametes, in the other,, megaga- 

 metes 1 result from the taking up of reserve food by 

 the individual cells of the colony. When tw-^ such 

 differentiated colonies, in their aimless wander igs, 

 bump into each other, they stick together, and the 

 microgametes of one colony separate, penetrate 

 the gelatinous envelope of the other, and fuse with 

 the megagametes there. Each resultant zygote, 

 after a resting period, divides into a sixteen- or 

 thirty-two-cell colony. 



In Volvox, a closely allied type, the differentiation 

 of the two kinds of gametes has reached a much 

 greater degree. The Volvox colony is composed of 

 a great number of individual units (as many as 

 22,000), which are not only united by the gelatinous 

 matrix in which they are immeshed, but by proto- 

 plasmic intercellular connections as well. In au- 

 tumn, a score or more of these cells begin to grow 

 by the accumulation of food reserves, until they 

 exceed the size of an ordinary cell many times 

 (see fig. 59). Other cells reverse the process and 

 divide repeatedly, until a great number of minute 

 microgametes result. The differentiation is not 

 alone one of size. In proportion as the megagametes 

 increase in bulk, they become immobile and lose 

 the two flagella with which the other cells are pro- 

 vided ; on the other hand, the cell- body of the micro- 

 gametes is markedly attenuated, and their activity 

 is correspondingly increased. The megagametes 



1 M^yas = large ; /IX/K/WS = small. 



