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, FIG. 29. Volvox: the large globular colony 

 and biciliate. Fertilization com P sed of sma11 vegetative cells connected by 



OCCUrs in the CavitV of the **T^ f Cyt plasm ' tw lar * e colony-forming cells, 

 Cavity tne an d 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 

 FIGS. 30-32. Volvox: 30, periph- to fa differentiation of sex. Volvox 

 eral cells of the colony (after WEST): . . . - . ,,. 



31, an egg surrounded by sperms; 32, an and lts colony-forming allies are to 



oospore with heavy wall. be regarded as specialized forms, and 



