The Role of the Chromosomes 



231 



shown, for the fertilized egg becomes either a queen or a 

 worker, according to the kind of food which it receives. 



In other parthenogenetic forms the sex relations are not 

 so clear. Among the fresh water Crustacea the daphnids pre- 

 sent a well-known example of parthenogenesis. When the 

 daphnids appear in spring or summer only females are found. 

 They lay thin-shelled eggs which develop without fertilization 

 into other females which in turn lay parthenogenetic eggs, 

 and so the story goes for several generations ; when suddenly, 

 males make their appearance, together with the "winter" or 

 "resting" eggs, which are fertilized, and surrounded by a 

 thick shell, in which they pass the winter inactive in the ooze 

 at the bottom of the water. "With the advent of warm 

 weather the following year these eggs hatch, giving rise to 



MOLTED SKIN AND LIBERATED EGG CASE OF A DAPHNID 

 Photo by Lloyd, from Needham and Lloyd's "Life of Inland 

 Waters/ 5 Comstock Publishing Company. 



parthenogenetic females, which repeat the story. The time 

 of appearance of males varies in different species. In some 

 it appears to be correlated with the lowered temperature of 

 autumn, and in others with the drying up of the pools in 

 which the animals live. A similar life history is presented 

 by the rotifers, or "wheel animalcules," so called from the 

 circlet of vibrating cilia at the anterior end. 



The group of nematodes or thread worms present some 

 curious modifications of the sex cycle. Some of these are 

 parasitic, others are free-living and yet others present a 

 so-called "alternation of generations" in which one genera- 

 tion lives parasitically and the other free. Moreover some are 

 hermaphroditic, others bi-sexual. Among the bi-sexual, free- 

 living forms, parthenogenesis is of common occurrence, having 

 gone so far that in one species, males occur only in the ratio 

 of 13 to 100,000 females, while in others they have not as 



