46 NATURAL SCIENCE [July 



of males, and he therefore concluded that the ' resting-eggs ' were 

 produced after fertilisation, while the ' summer-eggs ' (distinguish- 

 able by a difference in size into male and female eggs) were partheno- 

 genetic. Partly from the fact that very similar phenomena were 

 known in other groups, especially in the Cladocera, these views 

 obtained general acceptance, despite the fact that in his later papers 

 (8) their author found it necessary to point out some difficulties. 

 It had already been observed that each individual female produced 

 only one variety of eggs, male, female, or ' resting,' during her life- 

 time, and Cohn found spermatozoa in the bodies of females which 

 were laying male or female ' summer ' eggs, as well as in those lay- 

 ing ' resting ' eggs. Joliet, in his monograph on Melicerta (10), 

 corroborated these observations, and after giving a careful summary 

 of the evidence, suggested that the facts might be explained by sup- 

 posing that the sexually produced eggs hatched into females which laid 

 ' resting ' eggs. In 1885, Plate (12), in connection with his discovery 

 of ' hypodermic impregnation,' arrived at the strange conclusion that 

 impregnation was always abortive and without influence on the eggs. 

 He contradicted Cohn's statement that the spermatozoa in the body- 

 cavity of the female were attracted to the neighbourhood of the 

 ovary, but found, on the contrary, that they disintegrated within a 

 few hours after being introduced, without making any progress 

 towards reaching the ovary. By observing isolated specimens he 

 claimed to show that impregnation was without effect, male, female, 

 and resting-eggs being laid indifferently by impregnated and virgin 

 females. His explicit statement that he obtained resting-eggs from 

 a female isolated from birth is in direct conflict with the results of 

 later investigations. Plate concluded that uninterrupted partheno- 

 genesis prevailed among the Rotifera, that the males were a vestigial 

 and superfluous sex, and that the abortive attempts at impregnation 

 were an atavistic phenomenon without present significance in the 

 life-history of the species. In 1890, Maupas (16, 17, 19), attacked 

 the subject by applying to the rotifers the methods of culture used 

 by him in his classical researches on the reproduction of the Infu- 

 soria. Unfortunately only very brief accounts of his laborious 

 experiments have been published, and many details of his procedure 

 are not explained. He found in the species chiefly studied by him 

 (Hydatina senta) two varieties of females, distinguished only by the 

 eggs which they produced, one kind laying eggs which gave rise to 

 females, the other laying only male eggs. 1 The females of the latter 

 variety alone were capable of being successfully fertilised, and that 



1 It would be difficult to find an exact parallel to the curious 'sub-sexual ' difference 

 between these two classes of females. That it is not accompanied by any conspicuous 

 difference of size or structure is certain, but it would be interesting to know whether 

 careful measurements would not reveal a physical dimorphism correlated with the 

 physiological one. 



