LIFE-HISTORY OF ROTIFER 233 



organisms at different times in the history of the race suggested 

 the possibility of increasing these two factors still further by 

 artificial selection; an experiment with this in view is described 

 later. 



The early production of eggs, the large number of eggs pro- 

 duced, the high percentage of viabiUty in fresh malted-milk 

 solution, together with the short length of the average Ufe-cycle, 

 give us a race of organisms increasing rapidly. In 100 trials 

 made during the study, a single individual, isolated in a small 

 watch-glass, supplied daily with fresh malted milk and allowed 

 to reproduce for seven days, showed an average of 125.63 de- 

 scendants (adults, young, and eggs) , including members of four 

 generations. During the thirteen months the race has been under 

 observation over 125,000 ' individuals have been studied in 

 isolation; all of these organisms have produced eggs of the thin- 

 shelled variety, with the exception of the sixteen thick-shelled 

 forms mentioned earlier. The fact that only females had been 

 produced and the absence of the two types of eggs in the later 

 work led to a search for the male form and the experimental 

 conditions under which it appears, as set forth in the next section. 



DO MALES OCCUR? AN EXPERIMENTAL STUDY 



As has already been set forth in this paper, the line of descent 

 in Proales consists mainly of females reproducing partheno- 

 genetically. Is this the only method of reproduction? Do 

 males ever appear, as in most other species of rotifers? In a 

 number of species the male occurs either continuously or at 

 irregular intervals in the Hfe-history, but in one large group, the 

 Bdelloida, no males are known and reproduction takes place 

 continuously by parthenogenesis. In the genus Proales males 

 have been observed for two of the four species; in Proales wer- 

 neckii, which lives in galls of Vaucheria, the male resembles the 

 female closely in size and form, although the alimentary canal 

 is not so fully developed; in Proales parasita, Plate has recorded 

 the appearance of a male of the usual reduced size and structure; 

 in Proales decipiens and Proales gigantea the male has not been 

 recorded. 



