SEX-DETERMINATION IN ASPLANCHNA 227 



In typical parthenogenetic reproduction in rotifers it is plain 

 that all the individuals are female's^ and all the eggs which de- 

 velop into females are, in a true if somewhat special sense of the 

 word, female eggs. They must certainly contain the full anlage 

 of one type of female constitution. The mere fact that they do 

 not require fertilization for their development can not rob these 

 eggs of their character as female gametes any more than it can rob 

 the resulting parthenogenetic adults of their character as females. 

 In short in any series of purely parthenogenetic generations the 

 entire race is female. Now whenever this typical partlienogenetic 

 reproduction is so changed that certain eggs become male pro- 

 ducers or males, it seems plain to us that sex has been deter- 

 mined — namely that an exclusive female production has become 

 in part a male production. In point of fact, viewing the phenom- 

 ena more closely, we may see exactly what takes place: the 

 egg or reproductive cell of certain young females becomes modi- 

 fied (either in the course of one or two generations) so as to de- 

 termine — without fertilization — the production of a male. These 

 eggs therefore have become male eggs and whatever factor has 

 determined this, their character, is a factor of sex-determination. 

 It is true that these same eggs, in the event of fertilization, are 

 again redetermined as female eggs, becoming resting eggs, which 

 always hatch as typical females. It can not be too clearly under- 

 stood that the factor we are seeking is something which modifies 

 an egg, determining its inherent tendency towards the develop- 

 ment of a male or a female. It matters not in the least whether 

 we conceive the determining factor as acting directly upon the 

 egg in its earlier or later growth period or whether its influence 

 may have been indirect, having been cumulative through one 

 or more generations preceding its development. In a^y case we 

 have phenomena which it seems can appropriately be designated 

 as those of sex-determination. •■* 



- It is curious that in the discussion of sex-determination this fact seems usu- 

 ally forgotten or quite neglected, despite the fact that the same writers in all 

 other connections habitually designate parthenogenetic individuals as female. 



^ In advocating the view opposite to the one which we are here defending 

 Shull has claimed that the source of confusion in the case of rotifers lies in the fact 

 that there is no outward distinction between the parthenogenetic female and the 



