THALLOPHYTES 



twelve) much larger ones 

 which divide to form new 

 colonies. These large 

 colony-forming cells are 

 derived from the smaller 

 cells and have been called 

 gonidia, a very inappropri- 

 ate name. 



The sexual reproduction 

 is much as in Eudorina, 

 but the eggs become much 

 larger than the ordinary 

 cells and lose their cilia. 

 The sperms, produced by 

 the division of certain 

 cells, are elongated, yellow, 

 and biciliate. Fertilization 



FIG. 29. Volvox : the large globular colony 

 composed of small vegetative cells connected by 

 strands of cytoplasm, two large colony-forming cells, 



occurs in the Cavity of the and numerous oospores with rough walls. 



colony (fig. 31), and the 



resulting oospore is a resting, protected cell (fig. 32). Upon germina- 

 tion, there comes from the oospore a group of ciliated cells (equivalents 

 of zoospores) that represent a new colony. 



In this so-called Volvox colony differentiation has resulted in four 



distinct kinds of cells: ciliated vegeta- 

 tive cells, colony-forming cells, eggs, 

 and sperms. 



Conclusions. A summary of the 

 features of the Volvocales may be 

 stated as follows: The forms range 

 from isolated cells to complex spheri- 

 cal colonies, all the ordinary cells 

 being ciliate; a new colony is formed 

 from the division of a single mother 

 cell; sexual reproduction is present, 

 advancing from isogamy to heter- 

 ogamy, that is, from the origin of sex 

 to the differentiation of sex. Volvox 



FIGS. 30-32. Volvox : 30, periph- 

 eral cells of the colony (after WEST); 

 31, an egg surrounded by sperms; 32, an 

 oospore with heavy wall. 



and its colony-forming allies are to 

 be regarded as specialized forms, and 



