1906.] 



Sex-determination in Hydatina. 



227 



there can be little doubt of a purely thelytokous strain having arisen here. 

 Though arrenotokous ? 's are rare they yet average 2 to 5 per cent, in 

 families of Type B, and the fact of their non-appearance among 177 

 individuals seems to be fairly conclusive evidence of the inability of the 

 strain under consideration to produce them. 



Type C. Pure Thelytokous ? 's. 



Barer than Type B, but by no means difficult to obtain, are ? 's which are 

 incapable of giving rise to arrenotokous offspring. An example of such a 

 strain is shown in Fam. 3, in which 55 generations including 178 individuals 

 were bred and tested without the appearance of a single arrenotokous ? . 

 In another case 262 thelytokous individuals spread over 24 generations 

 were produced, whilst in a third instance (Fam. 4 of plate) the strain was 

 bred for]73 generations, during which 304 individuals were tested and shown 

 to be purely thelytokous. 



In view of these experiments L have referred to the ? 's of Type C as 

 purely thelytokous. By doing so I have no desire to ignore the possibility 

 that such strains may eventually " break " and give rise to some arrenotokous 

 $ s. Since, however, this has not occurred in my experiments the evidence 

 for regarding them as I have done is very strong. In any case the nature of 

 the ? 's of Type C must be distinct from those of Types A and B. 



Influence of Temperature and Nutrition. 



It might be argued by some that the continuance of purely thelytokous 

 strains was due to uniform conditions of temperature or of nutrition. Such, 

 however, is not the case. In one of the experiments a purely thelytokous 

 strain was subjected to considerable fluctuations of temperature. During 

 a period of eight days it varied between 8° C. and 11° C, averaging 9° C. 

 The rate of reproduction was much retarded, but in the many subsequent 

 generations produced when the temperature was raised no arrenotokous ? 's 

 made their appearance. The same strain was afterwards subjected for four 

 days to an average temperature of 22° C. (the extremes being 21° C. and 

 23° C.) without alteration of its purely thelytokous nature. 



A number of starvation experiments were also made, of which the following 

 may serve as an example. An egg from a purely thelytokous strain 



T (Starved for 6 hours after hatching.) 



1 ' ~l 



1 3 T T ^ Starved for 23 hours after hatching.) 



r 1 



5T T (Starved for 20 hours after hatching.) 



