Life Cycle of Hydatina Seuia 353 



If resting eggs are fertilized male eggs, and never fertilized 

 female eggs, male-producers are the sexual females. The appear- 

 ance of male-producers then becomes merely a transition from 

 the parthenogenetic to the sexual phase of the life cycle, and is 

 not different from that in certain aphids. In aphids it has been 

 shown (Slingerland, '93, and others) that external conditions may 

 influence the occurence of the sexual generation, and Issakowitsch 

 ('07) and Woltereck ('09) find the same true of daphnians. 

 There is a much larger body of earlier literature upon the subject, 

 but any discussion of this, or any attempt to relate the phenomena 

 in the rotifers to those in other groups, is purposely deferred until 

 mv data are more complete. In cases where the sexual female is 

 distinguishable from the parthenogenetic female, there has been 

 no confusion of the phenomena of the transition from one phase 

 to the other with sex-determination. Where, as in Hydatina, 

 the sexual is not externally distinguishable from the partheno- 

 genetic female, the inauguration of the sexual phase appears to be 

 merely the addition of males, hence the application of the term 

 "sex-determination" to the phenomena. The external similar- 

 ity of the two kinds of females does not alter the essential 

 nature of the case. Under this view, the interesting features of 

 the life cycle of Hydatina senta are that the sexual eggs may de- 

 velop without fertilization, in which case they produce males, 

 and that the two sexes of the sexual phase do not first appear 

 simultaneously. The sexual female always appears one genera- 

 tion earlier than the male, for she is, if unfertilized, the mother of 

 the males. 



Accepted by The Wistar Institute of Anatomy and Biology, March 6, igio. Printed June 22, 1910. 



