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- 
vocing 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 EHudorina 
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 Volvoz 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 Volvocine, 
