70 



PRINCIPLES OF ANIMAL BIOLOGY 



kinds, if the colonial theory of origin is correct — or a differentiation of the 

 parts of the cell which later became distinct cells, if the organismal 

 theory is correct. Following only the colonial origin, what differentiation 

 shall we expect? 



A B 



Fig. 55. — Pleodorina illinoisensis , consisting of 28 reproductive and 4 sterile cells. A, 

 young organism; B, reproductive stage. The sterile cells may be regarded as the beginning 

 of a soma. 



If we are to draw our answer to this question from the animals and 

 plants that live at present, we should look for those in which there has 

 been only one differentiation — in which, as a consequence, there are only 

 two kinds of cells. The only organisms which exhibit a single differentia- 

 tion among their cells are those in which some cells have lost the power of 

 reproduction, while others retain it. Pleodorina is an example. In one 





Fig. 50. Fig. 57. 



Fig. 56. — Pleodorina californica, with small sterile cells almost as numerous as large 

 reproductive ones. 



Fig. 57. — Volvox wcismannia, with 10 reproductive cells and thousands of sterile cells. 

 {From Powers, in Transactions of American Microscopical Society.) 



of its forms (Fig. 55), which may be only a variety of Eudorina elegans 

 (Fig. 52), it consists of 32 cells in a jelly matrix. Four of these cells, 

 placed at that side which moves foremost as the organism swims, are 

 smaller than the rest. These 4 cells are sterile, while the remaining 28 

 may reproduce. Any of the 28 larger cells may divide to form a group of 

 32 cells which escape from the jelly and lead an independent existence 



