164 FOOT-NOTES TO EVOLUTION. 
matic cells carry on their various functions for a time, 
grow old, die, and disappear, certain of the germ cells 
alone surviving in the production of new individuals. 
On the borderland between the unicellular and the mul- 
ticellular organisms, however, stand certain colonial 
forms, which show an exquisitely graded series of steps, 
from the conditions of unicellular multiplication to 
those of the multicellular forms. Let us examine a 
few examples of these. Pandorina morum is a minute 
fresh-water Alga, consisting of a colony 
of sixteen ovoid cells imbedded in a 
spherical mass of a jelly-like substance. 
From each of these cells two long, hair- 
like flagelle extend out freely into the water, and by 
their lashing to and fro the colony is propelled from 
place to place (Fig. 7, 4). In multiplication by simple 
division each one of these cells divides into a group 
of sixteen daughter cells, the general gelatinous inter- 
cellular substance of the parent colony dissolves, the 
sixteen daughter colonies become free, and by continu- 
ous growth soon attain the size of the parent colony 
(Fig. 7, 2). After a certain number of generations 
produced in this manner, the necessity for reproduction 
by conjugation ensues. In this method the sixteen cells 
of a colony divide, each one usually into eight minute 
cells, which are set free in the water by the dissolution 
of the common gelatinous envelope (Fig. 7, C). Each 
one of these swarm spores, or “ zoospores,” consists of 
an oval, greenish cell, the pointed end of which is hya- 
line and bears two long cilia, by means of which the 
spore swims through the water (Fig. 7, K). These zoo- 
spores are not all of exactly the same size, but no great 
difference is noticeable. If the zoospores from two 
different colonies come near each other, they unite in 
pairs made up of individuals of the same or of different 
Gradual differ- 
entiation of re- 
ductive cells. 
