632 THE PHYSIOLOGY OF REPRODUCTION 



cast doubts upon this view, believing rather that the animal 

 has been so constituted by natural selection that it tends spon- 

 taneously to reproduce sexually in the appropriate season, and 

 that it does so to a large degree irrespectively of the actually 

 existing conditions. More recently Issakowitsch ^ has carried 

 out. an investigation upon another daphnid, Simocephalus, 

 from which he has been able to show that differences in tem- 

 perature may determine the mode of reproduction, but that 

 this result is effected indirectly by the change of temperature 

 altering the conditions of nutrition. Unfavourable conditions 

 tend to the production of sexual forms, and favourable ones to 

 the parthenogenetic method of generation. The same individual 

 female may give rise either to sexual or parthenogenetic offspring, 

 the conditions which exist in the ovary appearing to determine 

 what kind of egg will develop. 



In the Rotifer Hydatina senta there are at least two kinds of 

 females, which are distinguished by the kinds' of eggs that they 

 lay — (1) thelytokous females, which produce other females 

 parthenogeneticaUy, and (2) arrenotokous females, which pro- 

 duce males parthenogeneticaUy. The second kind of female 

 may also produce fertiKsed eggs. Furthermore, the thelyto- 

 kous females may give rise either to arrenotokous females or 

 to more thelytokous females, and the proportion of arreno- 

 tokous females so produced is hable to considerable variation. 

 Maupas ^ has sought to connect this variation with differences 

 in temperature, and Nussbaum ^ with differences in nutri- 

 tion, but neither conclusion has been satisfactorily estabhshed. 

 The question has been reinvestigated by Punnett,* who has 

 carried out a number of further experiments. In one of these 

 a strain which had hitherto appeared to be purely thelytokous 

 was subjected to considerable fluctuations of temperature. The 

 rate of reproduction was much retarded, but in the subsequent 

 generations which were produced no arrenotokous females could 



' Issakowitsch, " Geschlechtsbestimmende Ursachen bei den Daphiden," 

 Biol. Centralbl., vol. xxv., 1905. 



' Maupas, toe. cit. 



" Nussbaum, "Die Entstehung des GescWechtes bei Hydatina senta,'' 

 Arch. f. Mikr. Anat., vol. xlix., 1897. 



* Punnett, "Sex-determination in Hydatina," Proc. Boy. Soc., B., 

 vol. Ixxviii., 1906. 



