30 ANIMAL LIFE 



divide into bundles of very small, slender cells, each of 

 which is provided with flagella. The remaining cells of 

 the colony (that is, those which have not swollen into egg 

 cells or divided into many sixty-four to one hundred and 

 twenty-eight minute, flagellate cells) remain unchanged for 

 a while and finally die. They take absolutely no part in 

 reproducing the colony. One of the minute free-swim- 

 ming cells fuses with one of the enormous egg cells, the 

 new cell thus formed being a resting spore. From this 

 resting spore a new colony develops by repeated division. 



18. Steps toward complexity. Within the group of Vol- 

 vocince there are plainly several steps on the way from 

 simplicity of structure to complexity of structure. Gonium, 

 Pandorina, Eudorina^ and Volvox form a series proceeding 

 from the simplest animals toward the complex animals. 

 In Gonium the cells composing the colony are all alike in 

 structure, and each one is capable of performing all the 

 processes or functions of life. In Pandorina and Eudorina 

 the cells are at first alike, but there is, as the time for 

 reproduction approaches, a differentiation of structure ; 

 the cells of the colony, all of which take part in the process 

 of reproduction, come to be in certain generations of two 

 kinds an inactive large kind which may be called the egg 

 cells, and a small, active, free-swimming kind which seeks 

 out and conjugates with, or, we may say, fertilizes the egg 

 cells. In Volvox there is a new differentiation. Only cer- 

 tain particular and relatively few cells take part in repro- 

 ducing the colony; most of the cells have given up the 

 power or function of reproduction. These cells, when the 

 time of multiplication comes, simply support the special 

 reproductive cells. They continue to waft the great colony 

 through the water by lashing their flagella ; they continue 

 to take in food from the outside. The reproductive cells 

 devote themselves wholly to the business of producing new 

 colonies, of perpetuating the species. And this matter of 

 reproduction is less simple than in the other Volvocince. 



