226 



Mr. E. C. Punnett. 



[May 15, 



I have consequently not made use of them in the following account. No 

 such doubt, however, attaches to the later experiments, for the advent of a 

 small centrifugal machine into the laboratory provided an easy and certain 

 means of obtaining a culture of Euglena free of rotifers and their eggs. 



Experimental Results. — My experiments have led me to the conclusion 

 that among the rotifers I used were certainly three different types of 

 thelytokous 9 's, viz. : — 



A. $ 's producing a high percentage of arrenotokous ? 's. 



B. ? 's producing a low percentage of arrenotokous ? 's. 



C. Purely thelytokous ? 's producing no arrenotokous ? 's. 



It will be convenient to consider the evidence for this conclusion under 

 these three headings. Incidentally it may be mentioned that the production 

 of S's is, under laboratory conditions at any rate, quite independent of the 

 season of the year. 



Type A. ? 's Producing a High Percentage of Arrenotokous ? 's. 



Only once was such a strain met with and bred over a series of genera- 

 tions. The result is shown in Fam. 2 (see diagram, Plate 11). It will be 

 noticed that from the seventh generation onwards the proportion of arreno- 

 tokous ? 's is high and bears to the thelytokous ? 's a ratio of about 7 : 9, the 

 actual numbers being 46 : 63. Here, as in other experiments, I was unable 

 to trace any order in the sequence of the two types of ? 's. The proportion 

 of arrenotokous ? 's among the earlier eggs of a family was neither higher 

 nor lower than among the later ones. 



Owing to an unfortunate failure in the food supply the strain was lost 

 before experiments had been made to determine whether it was capable of 

 giving rise to either type B or type C. 



Type B. ? 's Producing a Low Percentage of Arrenotokous ? 's. 



It has been my experience that if a ? is taken at random and bred from, 

 it will usually turn out to belong to this class. As reference to Fam. 1 

 (see plate) shows, the percentage of arrenotokous ? 's is low, but it is not 

 possible to determine it with any degree of accuracy unless very much 

 larger numbers are available. Even such figures may indicate but little, for 

 they may represent merely the resultant of more than one definite ratio. 



An interesting feature in this family is the production of an apparently 

 pure thelytokous strain. Commencing with T* we have a series of 15 

 generations, comprising 177 individuals, in which no arrenotokous ? 

 appeared. Failure of food then terminated the experiment, but I think 



