6;o THE PHYSIOLOGY OF REPRODUCTION 



and parthenogenetic females are not identical, and also that the 

 same female may give rise to parthenogenetic and sexual offspring, or 

 to males and females, or to only one sex. Moreover, Stevens has 

 shown that male and female embryos may be produced practically 

 simultaneously by the same individual. It is maintained, therefore, 

 by this writer that " the changes, in sex usually attributed to changes 

 in external conditions are really a change from the parthenogenetic 

 to the sexual mode of reproduction. The life cycle is often very 

 complicated, and in some species of Aphides there is evidence that 

 the environment (e.g. the trees on which they live) rather than the 

 temperature is responsible for the development of the sexual forms. 1 



Many of the lower Crustacea undergo a somewhat similar 

 alternation of generations. For example, the water-flea (Daphnia^, 

 after reproducing parthenogenetically during the summer time> 

 deposits eggs which give rise to sexual forms at the commencement 

 of autumn, and the female after impregnation lays the winter eggs 

 from which the new brood arises. This result is generally supposed 

 to be brought about by the conditions of temperature or nutrition. 

 But Weismann, 2 as a consequence of numerous experiments and 

 observations, has cast doubts upon this view, believing rather that 

 the animal has been so constituted by natural selection that it tends 

 spontaneously 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 3 has carried out an 

 investigation upon another daphnid, Simocephahis, from which he has 

 been able to show that differences in temperature 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 partheno- 

 genetic 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 partheno- 

 genetically, and (2) arrenotokous females, which produce males 

 parthenogenetically. The second kind of female may also produce 

 fertilised eggs. Furthermore, the thelytokous females may give rise 



1 Balbiani, "Le Phylloxera du Chene et le Phylloxera de la Vigne, etc.," 

 Mem. a UAcad. des Sri., vol. xxviii., 1884. Stevens, "Studies on the Germ Cells 

 of Aphides," Carnegie Institution (Washington) Publications, 1906. 



2 Weismann, " Beitrage zur Naturgeschichte den Daphniden," Zeitsck.f. wiss. 

 Zoologie, vols. xxvii., xxviii., xxx., and xxxiii., 1876-79. 



3 Issakowitsch, " Geschlechtsbestinmiende Ursachen bei den Daphiden," Biol. 

 Centralbl., vol. xxv., 1905. 



