310 Genetics of a Daphnia Hybrid during Parthenogenesis 



length. The length of the processes continues to increase relatively 

 to the length of the animal even after the adult stage is reached, so 



that the -^ ratio decreases with successive ecdyses. Thus in nine females 



of D. obtusa (clone H), measured both in first and second adult stages, 



the average -5 ratio in the first stage was 171 and in the second 14'4. 



Four females of the same species measured both in adolescent and first 

 four adult stages gave the following mean figures : 



Adolescent First Adult Second Adult Third Adult Fourth Adult 

 Ratio stage stage stage stage stage 



I 19-6 17-4 15-2 14-1 13-1 



- 1-85 1-91 2-23 2-20 2-22 



A 



It is thus clear that it is imperative that all animals should be 

 measured at the same stage of development, and the stage chosen was 



the second adult stage. Even with this precaution, the -^ ratio has the 



disadvantage that larger animals tend to have lower ratios than smaller 

 ones even of the same instar, showing that large animals have relatively 

 even larger abdominal processes. Since the size of the animal is very 

 greatly affected by changes of food, temperature, etc., these factors also 

 tend to obscure the genetic constitution of the animals as expressed in 



the o ratio. 



The -T ratio, however, does not suffer so much from this drawback. 

 A 



Although larger animals tend to have larger ratios than smaller ones 



of the same instar, the difference is not great. Consequently this ratio 



is less affected by environmental differences than the ^ ratio, and there- 

 fore has been the characteristic chiefly used (in the female). As we 

 shall see, however, even this feature is sufficiently sensitive to environ- 

 ment to obscure genetic composition to some extent. 



It will also be noticed from the above figures that the -j ratio 



reaches approximate equilibrium in the second adult stage, which, it will 

 be remembered, is the stage in which all females were measyred. 



As in the females, the abdominal processes of the males are relatively 

 very small at birth, and so it is necessary here also to make measurements 



