304 Genetics of a Daphnia Hybrid during Parthenoge^iesis 



Some Points in the Biology of the Cladocera which concern 

 the present experiments. 



The Cladocera reproduce both sexually and parthenogenetically. In 

 the former type of reproduction the egg forms two polar bodies in the 

 usual way. The fertilized egg always, so far as is known, develops 

 into a female. This female produces young parthenogenetically, and 

 these, if females, may do so again. Parthenogenetic reproduction may 

 thus be continued for a very large number of generations, and probably 

 indefinitely. One of my own lines of Daphnia ohtusa has been bred in 

 this way for over a hundred generations and is still in as strong con- 

 dition as at first. Sooner or later, however, sexual reproduction makes 

 its appearance by the production of males and of eggs requiring fer- 

 tilization. The males are produced parthenogenetically like the females, 

 and, like these, without reduction of chromosomes (Chambers, Taylor). 

 The appearance of sexual forms does not bring parthenogenesis to an 

 end, but the two methods continue side by side. Thus, for example, 

 a female may produce parthenogenetically a brood of females, then a 

 couple of eggs requiring fertilization (referred to hereafter as sexual 

 eggs), then returning to parthenogenesis it may produce a brood or 

 two of males, and then a few broods of females. In most of the lines 

 of Daphnia and Simocephalus which I have bred, parthenogenetic pro- 

 duction of females has been predominant, only a small percentage of 

 males and sexual eggs appearing now and then. 



As a rule, all the members of one parthenogenetic brood (at any rate 

 in Daphnia) are of the same sex, but mixed broods are not uncommon. 



The part played by the environment in determining sexual or par- 

 thenogenetic reproduction in these animals has frequently been made 

 the subject of experiment. This side of the question does not concern us 

 here, and it must suffice to say that it has been satisfactorily established 

 that, at any rate in the species with which we are dealing here, there 

 is no hard and fast predetermined reproductive cycle independent of 

 environment. 



Parthenogenetic eggs, whether destined to develop into males or 

 females, pass out of the oviduct into the brood pouch where they undergo 

 their development, escaping to the exterior four or five days after laying 

 (at room temperature). The number of parthenogenetic eggs laid may 

 vary from one to thirty or more, the number depending mainly on the 

 abundance of the food supply. Sexual eggs are however always produced 



