312 Genetics of a Daphnia Hybrid durmg Parthenogenesis 



measurement were : in the case of the % body length •0371 mm., </ body 

 length •0309 mm., and abdominal processes ^0062 mm. Before finding 

 the ratio between body length and abdominal process, the former had 

 therefore to be multiplied by 5'99 (counted as 6) in the % , and by 4*99 

 (counted as 5) in the c/". The lines along which the measurements were 

 taken are shown in Figs. 1 and 2. 



The mean values for these ratios in the clones chiefly employed are 

 shown in T^ble I. 



It will be noticed that the ratio ^ in the female differs but little in 



the two species. As in addition it has the drawback of being powerfully 

 affected by the same features of the environment as influence the general 

 size of the animal, no further use is made of this ratio in the $ in these 

 experiments. 



The production of the hybrids. 



In 1914, acting on experience acquired in the breeding of numerous 

 clones of Daphnids, I obtained specimens of Daphnia from a number of 

 different localities, and worked up a clone from a single parthenogenetic 

 female from each locality. All the clones employed were referable to the 

 two species S Daphnia obtusa (8 clones) and D. pulex (3 clones). Cross 

 matings were carried out between the two species, as well as control 

 matings within the species (both between clones, and between members 

 of the same clone). Every individual was examined at birth, and the 

 few males isolated from the females. In order to make a cross, an adult 

 female with young sexual eggs in her ovaries was placed with a male 

 until the eggs were laid into the brood pouch, after which the male was 

 removed for use with another female. The fertility of these matings can 

 be summarised as follows (Table II) : 



^ I am indebted to Mr D. J.,Scourfield for kindly confirming my identification of these 

 two species. 



