W. E. Agar 323 



actually smaller in body length than any clone H male measured — a 

 confirmation that their high ratios were due to a certain juvenility of 

 character rather than to an approach to the genetic constitution of the 

 D. obtusa males. It is also significant that there are no outstanding 

 variations towards the low ratio characteristic of the D. pulex parent. 



Finally — and most important of all — the testes of the specimen of 

 ratio 23"75 was examined and found to be of the usual hybrid type, con- 

 taining no spermatozoa. Unfortunately, I have no record of the condition 

 of the testes of the other aberrant male, since it died a few days after 

 measurement, before I had time to make the examination. 



It should be mentioned that both these aberrant males belonged to 

 otherwise normal broods. The (^ of ratio 23"75 had five normal brothers, 

 the highest ratio among them being 10"25 ; the cT of ratio 16*75 had two 

 brothers, of ratios 9"25 and 9*75. Thus if the high ratios of these two 

 males were due to segregation, this cannot have taken place earlier in 

 their ancestry than in the formation of the egg from which each 

 developed. 



Taking all these considerations into account — and to them may 

 perhaps be added the judgment formed by many years' experience of the 

 type of somatic modifications caused by environment in Cladocera — I 

 think conclusion unavoidable that two males of abnormally small size 

 and high ratios were not exceptions to the general rule of absence of 

 segregation obtaining in the rest of the clone, but merely examples of 

 extreme somatic modification. 



The conclusion to be drawn from this examination of the hybrid 



males is that no segregation of the factors which determine the -^ ratio 



has taken place during the course of this experiment. For ten partheno- 

 genetic generations (eight acts of reproduction) all males — and hence all 

 their female ancestors — have remained of the same heterozygous com- 

 position as that of the original F^ hybrid. 



The persistence of the characteristic of sterility in the males through- 

 out the successive generations of the hybrid clone is also of course strong 

 evidence against the occurrence of segregation. 



As we have seen, there were altogether ten generations of the hybrid 

 clone, counting the original hybrid female as generation 1, and 17 of the 

 female offspring from this hybrid were each used to found a line 

 (Table IV). In constructing the table to test the parental correlation 



