The Process of Heredity 507 



dimorphism would seem to be of no moment for the purposes of 

 this paper, since I have not as yet been able to rear hybrids to 

 maturity, yet a knowledge of this phenomenon is necessary as an 

 aid to understanding the methods employed. Under a subse- 

 quent heading is given a brief treatment of the essential points 

 about spawning behavior and sexual dimorphism. 



2 Physiological Differences Between the Adults 



a F. heteroclitus is markedly more resistant to adverse con- 

 ditions, such as foul water, lack of oxygen, presence of carbon 

 dioxide, etc., than is F. majalis. 



b F. heteroclitus is therefore found in habitats unfit for F. 

 majalis, such as brackish and foul ponds, etc. 



c F. heteroclitus is much less readily affected by confinement 

 in small aquaria and carries on its spawning in captivity with 

 perfect freedom. F. majalis, on the other hand, is rather sullen 

 in confinement and frequently refuses food for some time after 

 capture. Spawning in aquaria is very rare with F. majalis. 



d The Hesh is harder and the muscles stronger in F. majalis 

 than in F. heteroclitus. 



e The heart-beat, and hence the general circulation, is markedly 

 less rapid in F. heteroclitus than in F. majalis. 



These and other differences between the adults that might be 

 mentioned concern us much less directly than do differences 

 between the eggs and developing embryos of the two species. 



3 Morphological Differences Between the Eggs and Embryos 



a The eggs of F. majalis are decidedly larger than those of 

 F, heteroclitus, the average diameter of the former being 2.7 mm. 

 and that of the latter 2 mm. A calculation shows that the vol- 

 ume of the average F. majalis egg is over twice that of the average 

 F. heteroclitus ego-. 



b The eggs of F. majalis are of a decidedly yellowish color, 

 while those of F. heteroclitus are almost colorless and more 

 nearly transparent. These differences are due to a different com- 

 position of yolk and protoplasmic content. 



