316 Genetics o/a Daphnia Hybrid during Parthenogenesis 



It is curious, in view of the imperfect fertility of the parthenogenetic 

 eggs, and of the complete sterility of the hybrid males, to be described 

 below, that the sexual eggs produced by the hybrid females seem to be 

 perfectly fertile when these are mated to males of the two parental 

 species. These matings give about the same percentage of fertilized 

 eggs as matings between pure bred members of the same species. This 

 is shown by Table III, which should be compared with Table II. The 

 criterion for, deciding whether an egg has been fertilized is the same 

 as in Table II, namely the fact that it becomes rounded off and takes up 

 its proper position in the ephippium. Up to the time of writing, none 

 of these eggs had hatched. 



TABLE 111. 



Fertility of Matings with Hybrid Females. 



Number of Number of Percentage of 

 successful unsuccessful successful 

 Type of mating matings matings matings 



Hybrid? xD. obtiisa^ ... 44 8 86 



Ryhrid Q xD. pulex^ ... 24 6 80 



Finally, the hybrid clone exhibited the important feature of complete 

 sterility of the males. This is a phenomenon which has often been 

 recorded in interspecific crosses, even where the females are fertile ; for 

 references the reader is referred to Detlefsen. 



In the normal fertile male of the parent species, the testes are 

 easily seen through the transparent tissues of the living animal, and 

 when mature the refractive spermatozoa can be seen floating in a fluid 

 which flows up and down in the tubular testis with the motions of the 

 animal's abdomen. The testes of the living hybrids present quite a 

 different appearance. Though of the same size and distinctness, they 

 appear solid, and no spermatozoa are visible. Many males, belonging 

 to different generations of the hybrid clone, were sectioned, and the 

 absence of spermatozoa was confirmed by microscopic examination. The 

 spermatogenesis of these hybrids is being worked out by another in- 

 vestigator, and at present it must sufl&ce to point out that the full size 

 of the hybrid testis, combined with the absence of spermatozoa, indicates 

 that spermatogonial mitosis proceeds successfully, but that the meiotic 

 phase is not completed. 



Nine males were tested with females. They copulated freely, with 

 the result that the eggs were laid, but all disintegrated soon after, 

 showing that they had not been fertilized. 



Summing up, the clone, or group of individuals obtained partheno- 

 genetically from the hybrid female Xc, showed the following features in 



