56 



THE SMALLEST LIVING THINGS 



nies, in the majority of species, being made up of a definite num- 

 ber of cells grouped in definite arrangements. Thus there are 



Fig. 27— COLONY FORMS OF GREEN FLAGELLATES 



A, a colony of eight cells, Stephanosphaera pluvialis, after Kiihn; B, a colony of 16 cells, 



Spondylomorum quarternarium, after Stein; C, a colony with two kinds of cells (the small cells 



are vegetative cells, the large ones are germinal), Pleodorina californica, after Chatton 



Magnifications: A and B, 500; C, 120 



eight cells in some, sixteen in others, thirty-two in others, etc. 

 Fig. 27). In Pleodorina from Illinois (Pleodorina illinoisen- 

 sis) there are thirty-two cells in the colony, but in the form from 

 California (Pleodorina californensis) , however, there are many 

 more than this number, those in one hemisphere of the colony 

 being much larger than those in the other (Fig. 21c). Each 

 individual of a sixteen-cell colony of Gonium pectorale may repro- 

 duce the entire colony by continued division (Fig. 28), but in 

 Pleodorina certain cells only have this power, the remainder dis- 

 integrating. In Volvox colonies, consisting of many hundreds 

 of cells, certain cells (parthenogonidia) withdraw from the pe- 

 riphery and produce other colonies within the jelly sphere 

 (Fig. 26). 



Sexual phases reach a high development in these types. In 

 the majority of forms the male and female gametes are similar 

 in shape and size. In Eudorina practically all of the thirty-two 

 cells may form gametes, termed in this case anisogametes (un- 

 equal gametes), since some of the cells form larger gametes 

 which serve as eggs, while others form smaller gametes which 

 serve as sperms. In Volvox only a limited number of the cells 

 form gametes, a fact which, together with the presence of vege- 

 tative cells and of parthenogonidia, or colonies that reproduce 

 asexually, indicates a relatively high division of labor (see 

 page 10). 



