244 EDITH PINNEY 



origin of the nucleus. In the hybrid blastoderms the paternal 

 nucleolus is lacking. Reference has been made above to the 

 demand upon these hybrid eggs for a larger production of chro- 

 matin than is necessary in the normal Ctenolabrus eggs. The 

 idea was advanced that this increased production was due to 

 the presence in the egg of larger chromosomes than it normally 

 contains and that this synthesis of extra chromatin leads to an 

 early exhaustion of the necessary substances, so that in later 

 stages the foreign chromatin is not able to function normally. 

 The single nucleolus present in the hybrid egg at a stage when in 

 normal eggs there are two may be regarded as an indication of the 

 failure of the paternal chromatin to continue development. 



I wish to point out the possible correlation between the bi- 

 nucleolate condition and the greater viability of the Fundulus 

 and Menidia hybrids in contrast with the mononucleolate con- 

 dition and the failure to survive gastrulation of the Ctenolabrus 9 

 X Fundulus d^ hybrids. Further study of hybrid nucleoli is 

 necessary to decide whether or not such a correlation is of uni- 

 versal application. 



Ctenolabrus adspersus X 9 Stenotomus chrysops cf 



No direct evidence as to the number and form of the chromo- 

 somes of Stenotomus was obtained. It is inferred, however, 

 from a study of the egg of Ctenolabrus fertilized with the sperm 

 of Stenotomus that the chromosomal complex of the latter species 

 closely resembles that of Ctenolabrus. Figure 50 represents 

 an anaphase spindle of the first cleavage mitosis in this hybrid. 

 It is very sbnilar to figures 9 and 10 which show anaphases from 

 pure Ctenolabrus eggs. Since the experiments yielding this 

 hybrid material were well controlled, there can be no question 

 of its authenticity, although the chromosomes contributed by the 

 two parents cannot be distinguished morphologically. Similarly, 

 an egg of Stenotomus fertilized with the sperm of Fundulus hetero- 

 clitus should show ver}^ much the same sort of chromosomes as 

 the egg of Ctenolabrus when crossed with Fundulus, figures of 

 which are referred to in the previous section. Figures 55 and 



