THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 3] 
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. 
19. Individual or colony.—Is the Gonium colony, the 
Pandorina colony, or the Volvoz 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 Fu- 
dorina, while all the cells are for most of the lifetims 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 tle 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 Volvo: 
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 re productive cells. 
The great majority of the cells take no part in reproduc- 
