OTHER METHODS OF SEX-DETERMINATION 233 



produces a female ; an egg fertilized by a sperm with five 

 chromosomes produces a male. Here an accident in cell- 

 division becomes the mechanism of sex-determination. 



Diploid Females and Haploid Males. 



In the rotifers there is, first, a long series of genera- 

 tions of parthenogenetic females with the diploid number 

 of chromosomes. No reduction takes place in the eggs and 

 one polar body is given off. The series may apparently 

 continue indefinitely under certain conditions of nourish- 

 ment. The series can, however, as shown by Whitney, bo 

 brought to an end by a change in diet — such as feeding 

 the females on a green flagellate. A female feeding on 

 such a diet now produces daughters (by parthenogenesis) 

 with dual possibilities. If one of these daughters is fertil- 

 ized by a male (that may have then appeared), each egg, 

 before maturation, is entered by a single sperm. The egg 

 enlarges in the ovary and a thicker shell is laid over it 

 (Fig. 137). It gives off two polar bodies, and then the 

 sperm nucleus (haploid) unites with the haploid nucleus 

 of the egg, restoring the full number of chromosomes. 

 This egg is a resting or winter egg. It contains the dip- 

 loid set of chromosomes, and after a time develops into 

 the stem mother of a new line of parthenogenetic females, 

 etc. 



On the other hand, if the female in question is not fer- 

 tilized, she produces eggs that are smaller than the ordi- 

 nary parthenogenetic eggs. The chromosomes conjugate, 

 and two polar bodies are given off. The egg is left with a 

 haploid set of chromosomes. It segments, without dou- 

 bling the number of its chromosomes, and produces a male. 

 It is not clear what happens during the formation of the 

 spermatozoa in the haploid males. Neither Whitney's 

 (1918) nor Tauson's (1927) work gives a convincing ac- 

 count of the changes that take place. 



