314 Genetics of a Daphnia Hybrid during Parthenogenesis 



period. The ephippium was in a small tube containing only two or 

 three cubic centimetres of water, standing in my laboratory. Unfor- 

 tunately, this animal died when about a week old, owing to fungoid 

 growth. 



The hybrids to be described below were all derived from a fresh set 

 of crosses carried out in the year 1919. Some 21 fertile ephippia from 

 crosses between the same two species, and 22 from straight fertilizations 

 were obtained in the early part of that year, and one or both of the eggs 

 in eight of these had hatched by May, giving eleven young. The interval 

 which elapsed between fertilization and hatching in these eggs varied 

 from six to sixteen weeks. The young hatching from these ephippial 

 eggs experienced a very high mortality, no less than seven dying before 

 their first ecdysis, and one other as a young adult. A summary of these 

 eleven young and their fates is given below. 



Number 

 Type of mating of young Fate 



D. ohtusa X D. obtusa 1 died as young adult 



D. pulex X D. pulex 7 6 died young, 1 gave rise to clone RR 



D. obtusa X D. pulex 2 1 died young, 1 gave rise to clone Xc 



D. pulex X D. obtusa 1 gave rise to clone Xf 



By far the greater part of the work was done with the hybrid 

 clone Xc, and this we will now describe. 



Genetics of the hybrid clone Xc. 



The origin of this clone was, then, the mating D. obtusa (clone H) 

 X D. pulex (clone R). On attaining maturity, the hybrid ? immediately 

 showed one of the most striking characteristics of the hybrid clone to 

 which it gave rise, namely a marked derangement of its reproductive 

 processes. Thus the history of its first three broods is as follows : 



March 30. Six parthenogenetic eggs in brood pouch. 



April 2. Five of the eggs broken up, one has developed into an embryo. 



April 5. This embryo (a (J) born. Parent has four more parthenogenetic eggs 



in the brood pouch. 

 April 7. Two of these eggs broken up. 

 April 9. One living young only born (a (J). Parent has ten more eggs in the 



brood pouch. 

 April 10. All eggs dead except one. 

 April 11. One $ born. Parent has fourteen eggs in brood pouch (4th brood). 



After this she produced a number of broods (some (/" and some ^ ) more 

 successfully, with only a small proportion of eggs failing to develop. 



The two peculiarities of this history are of course the early production 

 of (/"s and the large proportion of eggs which failed to develop. A third 

 peculiarity made its appearance among the daughters of the hybrid — 



