38 MANUAL OF ANIMAL BIOLOGY 



which enables them to withstand the lower temperatures of the 

 winter season. In the following spring, when the water warms up, 

 the zygote begins to divide and soon develops into a typical Volvox 

 colony. It appears, therefore, that the sexual method of reproduc- 

 tion, with the consequent zygote formation, enables the organism 

 to survive unfavorable environmental conditions. 



TEXTBOOK BEFERENCES 



Woodruff, pp. 57-61. 



Curtis and Guthrie, pp. 195-201. 

 Guyer, pp. 191 ; 333-338 ; 663. 

 Hegner, pp. 44-45 ; 72-86. 

 Newman, pp. 81-89 ; 146-152. 

 Shull, pp. 70-79 ; 147-148. 



GENEBAL BEFEBENCES 



Calkins. Biology of the Protozoa (Lea and Febiger). 



Hartog. " Protozoa," in the Cambridge Natural History (Macmillan). 



Kudo. Handbook of Protozoology (Thomas). 



Parker and Haswell. Textbook of Zoology (Macmillan). 



