638 THE PHYSIOLOGY OF REPRODUCTION 



possessed sometimes by the male character and sometimes by 

 the female. No zygotic individual is of either sex purely, for 

 the characters of the recessive sex (whether it be male or female) 

 are latent, as has been shown both anatomicaUy and experi- 

 mentally. 1 In parthenogenetic animals, however, the female 

 character always dominates over the male whenever the 

 characters of both sexes are present together. In such species, 

 all the fertilised ova are female, those unfertilised ova which 

 are formed without the segregation of the sex characters are 

 also female, while male individuals only develop from un- 

 fertilised ova from which the female character has been 

 eliminated. 



" The segregation of sex characters takes place in most 

 parthenogenetic animals, and doubtless in dioecious animals 

 also, at the second maturation division (the ' reduction 

 division ') of the egg, and probably at a corresponding stage 

 in spermatogenesis. For (1) eggs which develop without ferti- 

 lisation and without undergoing a second maturation divi- 

 sion contain both the male and female characters, the former 

 recessive, the latter dominant ; but (2) in normally partheno- 

 genetic species, eggs which, after undergoing a second matura- 

 tion division, develop without fertilisation, are always male 

 (except in the gall-wasp, Rhodites). In such species the female 

 character regularly passes into the second polar cell, the male 

 character remaining in the egg. In dioecious animals, on the 

 other hand, either sex character may remain in the egg after 

 maturation." 



In Hydatina senta only a single division occurs in the matura- 

 tion of the male eggs, and this is held to be comparable to the 

 second maturation division of other parthenogenetic forms, 

 and in it a segregation of sex characters is believed to take 

 place. In the case of the female parthenogenetic ovum no 

 maturation division occurs. The parthenogenetic egg of the 

 gall- wasp (Rhodites rosa) undergoes two maturation divisions, 

 but apparently without segregation taking place in either of 

 them, for no reduction occurs (at least normally), the nucleus 

 of the ovarian egg, the three polar nuclei, and the nuclei of the 

 mature egg being alike in each containing nine chromosomes. 

 1 Evidence that this is so is given below (p. 652 tt seq.). 



