192 A. FRANKLIN SHULL. 



paper (Shull, '15). Complete families were isolated from early 

 in November to December 15, 191 2, but no males appeared. 

 During the time when periodicity of male production was being 

 studied, representative mass cultures were reared. These cul- 

 tures showed males in small numbers only, and at times separated 

 by wide intervals. The times of male production, as far as 

 known, are shown in Table III., though the proportion of males 

 was not recorded. 



The periods of male production in the Nebraska line are 

 separated by intervals of three to five months, the interval 



Table III. 



Showing Times at Which Males Were Produced in a Nebraska Line of 



Hydatina senta. 



Number of Period of Male Production. Dates Between Wfiich Males were Found. 



First January 15 to February i, 1913 



Second April 22, 23, 1913 



Third August 10 to 16, 1913 



Fourth December 7 to 11, 1913 



Observation wanting^ 



Sixth(?)2 November i, 1914 



increasing with the age of the line. Though no complete families 

 were reared, during any of these "waves" of male production, 

 it was evident that in the later periods there were fewer males 

 than in the earlier ones.^ 



In each of the three lines described, there was a well-marked 

 rhythm in the production of males. The great regularity of 

 this periodicity, especially in the first two of these lines, and the 

 fact that the lines differed considerably from one another in 

 regard to the interval between periods of male production, forbid 

 the assumption that the waves of male production were brought 

 on by specific external conditions. 



' Observations were wanting from March 8 to June 10, 1914. Probably not 

 more than one period of male production fell between these dates, hence the next 

 males recorded are to be regarded as belonging to the sixth period. 



2 A single male-producing female appeared in this "wave" of male production. 



^ In the English line described in Table II., while the intervals between periods 

 of male production did not increase, with the age of the line, the number of males 

 in successive periods plainly decreased. This is a confirmation of a conclusion 

 which I formerly drew from lines bred through shorter periods, namely, that the 

 proportion of male-producing females gradually decreases with the age of the line. 

 Whether, as Mitchell suspects, this decrease is due to uniformity of conditions, is 

 a question not answered by the evidence. 



