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 

 Gradual differ- ^j sixteen ovoid cells imbedded in a 

 entiation of re- .1 r • n 11 , 



ductive cells. spherical mass of a jelly-like substance. 



From each of these cells two long, hair- 

 like flagellse extend out freely into the water, and by 

 their lashing to and fro the colony is propelled from 

 place to place (Fig. 7, A). 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, j5). 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 



