DEPARTMENT OF EXPERIMENTAL EVOLUTION. 119 



conditions found in normal females. The significance of this lies in the 

 fact that it is in the wide (generic) crosses that we are able to obtain 

 males from eggs of initial female-producing tendency. In other words, 

 additional evidence is thus obtained for the reaUty of sex-control in the 

 generic crosses; and in this group of male hybrids the gonads frequently 

 retain or approximate to the gonad size and weight relations of the 

 female. 



Dr. Riddle has obtained two pairs of identical twins among the 

 pigeons. Both were pairs of females. He says: 



"The importance of the finding lies in the fact that the storage metabolism 

 of the germs (ova) which produced these pairs of twins is known to have been 

 higher than in the other ova which arose from the same ovaries and which 

 produced only single (not twin) embryos. It seems that similar information 

 concerning the germ from which such twins have arisen in any other animal 

 forms has never hitherto been obtained. In addition, the fact that the two 

 twin-producing ova were of high storage value, and both produced females, is a 

 further confirmation of my views as to the metabolic basis of sex." 



SEX DETERMINATION IN CLADOCERA. 



Dr. Banta has made marked progress in his studies of sexuality in 

 Daphnia and related genera. He reports as follows: 



"With my work the most important and most desired advance of the year 

 has been in the further light obtained as to the factors controlling sex in 

 Cladocera. The general and long-standing behef has been that there is a 

 periodicity in the occurrence of sexual forms in Cladocera and that this is con- 

 trolled by internal factors. In former reports (1915 and 1916) I pointed out 

 that the long-continued and uninterrupted parthenogenetic reproduction in 

 the five species being bred in the laboratory did not result in reduced vigor 

 or in any evidence of need for sexual reproduction. I also pointed out that 

 in the few isolated cases in which males had appeared in the laboratory cul- 

 tures there was evidence of changed environmental conditions as the causative 

 factors. During the past year overwhelming evidence, it seems to me, has been 

 obtained that environmental factors induce the occurrence of sexual forms." 



Evidence from the observed incidence of males. — It may be safely stated 

 that with our methods of handling and observing the Cladocera material 

 few males occur without our knowledge; yet for a period of 5| years 7 

 strains of Daphnia pulex passed through, on the average, more than 225 

 generations in the laboratory cultures without producing males or 

 sexual eggs. For a lesser period of time, but throughout 186 genera- 

 tions, of 10 strains of Simocephalus vetulus no males or fertihzable eggs 

 were produced (except under the special conditions obtaining in the 

 sex-intergrade strain). Of 5 Unes of Moina 150 generations and of 3 

 lines of Simocephalus serrulatus 65 generations did the same. During 

 146 generations of Daphnia longispina males had occurred only twice 

 and then only in very small numbers. Additional strains (many of 

 which are no longer being bred) of all the above species have been in the 

 laboratory for various lengths of time. None of them produced males. 



