THE LIFE OP THE SLIGHTLY COMPLEX ANIMALS 31 



At least there is much more difference between the two 

 kinds of reproductive cells. The egg cells are compara- 

 tively enormous, and they are stored with a mass of food 

 material. The fertilizing cells are very small, but very 

 active and very different from the egg cells. We have in 

 Volvox the beginnings of a distinct division of labor and 

 an accompanying differentiation of structure. Certain 

 cells of the colony do certain things, and are modified in 

 structure to fit them specially for their particular duties. 

 The steps from the simplest structure toward a complex 

 structure are plainly visible. 



'lO^Individual or colony. Is the Gonium colony, the 

 Pandorina colony, or the Volvox colony a group of several or 

 many distinct organisms, or is it to be considered as a sin- 

 gle organism ? With Gonium, which we may call the sim- 

 plest of these colonial organisms, the colony is composed 

 of a few wholly similar cells or one-celled animals, each 

 fully capable of performing all the life processes, each 

 wholly competent to lead an independent life. In fact, 

 each does, for part of its life, live independently, as we 

 have already described. In the case of Pandorina and Eu- 

 dorina, while all the cells are for most of the lifetime of the 

 colony alike and each is capable of living independently, 

 at the time of reproduction the cells become of two kinds. 

 A difference of structure is apparent, and for the perpetua- 

 tion of the species the co-operation of these different kinds 

 of cells is necessary. That is, it is impossible for a single 

 one of the members of the colony to reproduce the colony, 

 except for a limited number of generations. With Volvox 

 this giving up of independence on the part of the individual 

 members of the colony is more marked. There is a real in- 

 terdependence among the thousands of cells of the colony. 

 The function of reproduction rests with a few particular 

 cells, and for the perpetuation of the species there is demand- 

 ed a co-operation of two distinct kinds of reproductive cells. 

 The great majority of the cells take no part in reproduo 



