68 



Heredity and Eugenics 



BEE 



ROTIFER APHID 



ment unless fertilized, but those of the hymenoptera may 

 develop either fertilized or unfertilized. In the former 

 case a female is produced, in the latter a male. The simplex 

 or X condition is in this case the male, the duplex or iX 



condition is the female, 

 naturally the one of higher 

 metabolic activity, the one 

 which forms the macro- 

 gametes. 



There is a peculiarity in 

 the maturation of the sperm 

 cells of male animals of this 

 sort (Fig. 2>i-: lower row). 

 The cells of the male are in 

 this case already in the re- 

 duced or simplex condition, 

 N. In the production of 

 the sperms the reducing 

 division is omitted so far as 

 nuclear components are 

 concerned, so that each 

 sperm formed contains the 

 full simplex chromosome 

 number, N. If it were less, 

 the gamete formed would 

 perhaps not be capable of transmitting all the hereditary 

 characteristics of an individual. 



A second category of cases (Fig. 34, middle column) 

 is represented by such simple aquatic organisms as rotifers 

 and small Crustacea, like Daphnia. In these partheno- 

 genesis occurs exclusively, when the food supply is very 

 abundant and conditions otherwise favorable, whereas 



Fig. 34. — Sex determination in par- 

 thenogenesis. Top row, nuclear condition 

 of the parthenogenetic mother; second 

 row, of her eggs when they develop with- 

 out nuclear reduction, having formed a 

 single polar cell; third row, condition of 

 the eggs after complete maturation — the 

 unfertilized egg in each case produces a 

 male; fourth row, nuclear condition of 

 the fertilized egg, always a female. 



