PERIODICITY IN THE PRODUCTION OF MALES. IQI 



In ano ther line, bred for a much longer time than was the line 

 described above, there was equally clear evidence of regular 

 periodicity, though the extent of the waves of male production 

 was not determined. A line of rotifers obtained from England 

 in the fall of 1912, described in another paper (Shull, '15), has 

 been reared up to the present time. No considerable numbers 

 of individuals of this line have as a rule been isolated, hence the 

 sex^ratio can not be stated; but during the time when the 

 periodicity of other lines was being examined, several dozen 



Table II. 



Showing Dates between Which Males Were Produced in an English Line 



OF Hydatina senta. 



No males appeared between one period of male production and the next. 



Number of Period of Dates Between Which Males 



Male Production. Were Found. 



First February 13 to 25, 1913 



Second April 17 to 26, 1913 



Third June 19 to 24, 1913 



Fourth August 24 to 30, 1913 



Fifth October 31 to November 15, 1913 



individuals of most generations were reared in two or three 

 dishes. As the number of males was always small in this line, 

 males were found in these "mass" cultures only occasionally. 

 They nearly always appeared in the dishes containing several 

 successive generations, and then were wanting for a considerably 

 longer period. There was thus a rhythm of male production, 

 which, as shown in Table II., proved to be fairly regular and 

 definite, though the number of male-producers in each period 

 was not known. 



The interval between periods of male production was, in this 

 line, a trifle over two months. Inasmuch as this line was bred 

 in part simultaneously with the one recorded in Table I., the 

 difference in the interval of male production in the two lines 

 (one month in the first, over two months in the second) effec- 

 tually disposes of any suspicion that this rhythm was induced 

 by external conditions. 



A third line which showed evidence of periodicity in the produc- 

 tion of males was obtained from Nebraska in 1912, and has 

 been reared ever since. This line is also described in a recent 



