150 GENERAL BIOLOGY 



the former is a member, and the resulting zygote, 

 becoming encysted, either develops directly into a 

 " daughter-colony," or winters over in a cyst, to 

 produce a new colony the next spring. In such a 

 case, the multitude of the swimming cells composing 

 the original colony perish. We see here a funda- 

 mental difference between a colony such as Eudorina, 

 in which each member is capable of functioning as 

 a gamete, and of reproducing the race, and Vol- 

 vox, in which only a very few of the cells so function, 

 and the great majority perish with each generation. 

 This condition is an accompaniment to a form of 

 specialization, in which certain cells of the organism 

 become peculiarly adapted for the function of repro- 

 duction. When the specialization has gone on so far 

 as is the case in Volvox, we usually speak of the large 

 passive megagamete as an egg, and the tiny active 

 microgamete as a sperm. 



Those cells of the aggregate that exercise the 

 reproductive function we speak of as germ-cells, in 

 contrast to the somatic cells, which have lost this 

 function. The differentiation has thus affected the 

 character of the protoplasm in the two kinds of 

 cells, one kind the germ-plasm, in which the func- 

 tion of reproduction is especially developed, is, 

 from one point of view, immortal, whereas the 

 soma-plasm dies with each generation and is renewed 

 with each generation by growth from the germ- 

 plasm. 



