306 Genetics of a Daphnia Hybrid dti7^ing Parthenogenesis 



An ephippium is always formed when sexual eggs are produced in 

 the ovaries, whether these eggs are laid or not. In the latter case the 

 ephippium is cast empty after the usual interval has elapsed since 

 the time at which oviposition should have taken place. 



A feature of great importance for an experiment in which bodily 

 dimensions are concerned is that the Cladocera, like so many other 

 Arthropods, only increase in size at each ecdysis. Between two ecdyses 

 their dimensions do not undergo any change detectable by the micro- 

 meter. The ecdyses and mode of growth etc. of the Cladocera was 

 described in 'some detail in my former papers (especially 1913), and 

 need not be redescribed here. During the interval between the last 

 ecdysis before the first batch of eggs is laid and the ecdysis which 

 immediately precedes the first oviposition, the animal is said to be in 

 the adolescent instar or stage. Between the last mentioned ecdysis and 

 that which immediately precedes the laying of the second batch of eggs, 

 the animal is in the first adult instar or stage. Then follows the second 

 adult instar, and so on. 



Culture methods. 



The methods used for culture were in the main the same as those 

 employed before and described in my previous papers. For food however 

 I used a culture of green flagellates — mainly Euglena and Chlamydo- 

 monads — grown in a solution made by boiling 25 gms. of canary guano 

 in 11 litres of water and afterwards filtering. These cultures were kept 

 in large battery jars and provided an inexhaustible supply of food. After 

 a few months, the bottoms of these culture jars became carpeted by 

 encysted organisms to a depth of several millimetres. 



Another departure from the methods previously described by me 

 was that, instead of isolating each individual at birth, up to about 

 six members of each brood, verified to be of the same sex, were generally 

 placed immediately after birth in one tumbler and kept there till 

 maturity, when they were isolated into separate 4" x 1" breeding tubes. 



The experimental data. 



Material and Characters employed. 



My first experiments (1908) were attempts at intergeneric crosses 

 between Daphnia and Simocephalus, but no fertilized eggs were obtained. 

 Indeed, no attempt at copulation was ever observed. The experiments 

 now to be described concern species crosses within the genus Daphnia. 



