TIIALLOIMIYTKS 



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Fig. 20. — Volvox: the large globular colony 

 composed of small vegetative cells connected by 

 strands of cytoplasm, two large colony-forming cells, 

 and numerous oospores with rough walls. 



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 



gonidla, 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 



occurs in the cavity of the 



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 

 and its colony-forming allies are to 

 be regarded as specialized forms, and 



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. 



