86 



FRESHWATER ANIMALS 



succession of individuals is apparently maintained by a failure 

 of the second reduction division during maturation of the oocyte 

 nucleus. The eggs so produced are often referred to as "partheno- 

 genetie" eggs, and although in some exceptional situations (Ed- 

 mondson, 1955) this designation loses its utility, it is sufficient 

 for present purposes. This derivation of one generation of females 



Fig. 1. Diagrammatic representation of the modes of reproduction in 

 Daplinia. Usually reproduction is uniparental and all the individuals arc 

 females. Occasionally, however, the females produce eggs requiring fer- 

 tilization, and more or less simultaneously some of the parthenogenetie eggs 

 develop into males instead of females. Females bearing ephippia and 

 "sexual" eggs, and males usually appear just after population maxima. Eggs 

 (actually early embryos) shed in the ephippium are in a dormant state and 

 they can often withstand drying and freezing. If conditions improve, the 

 female can resume parthenogenetie egg production after she has shed an 

 ephippium. Males are short-lived. 



from the previous by diploid parthenogenesis is represented dia- 

 grammatically in Fig. 1 by the two females in the right half of 

 the diagram. As long as the food supply remains adequate, the 

 female generations will succeed one another parthenogenetically 

 lor an apparently indefinite 1 period (Banta, 1939). Shortly after 

 each molt, the parthenogenetie eggs are extruded into the space 

 between the body proper and the dorsal part of the bivalve cara- 

 pace. The) are prevented from falling out of this space, referred 



