cc6 H . H. Neivnmn 



have a blending both in the time of establishment and the rate 

 of the heart rhythm. The M hybrids acquire a circulation at 

 least twelve hours earlier than the M pures, but it is a slower and 

 less efficient circulation. For a time the M hybrid gains markedly 

 upon the M pure, but the slower heart rhythm seems to make it 

 impossible for the circulation to incorporate more than about half 

 of the yolk and while the M pures go ahead and develop into large, 

 normal young fish the M hybrids reach a stage as advanced as 

 that at which the embryos of other strains hatch, but are left 

 stranded on a mass of yolk that the metabolic processes are unable 

 to cope with. The fact that the size of these abnormal embryos 

 is about equal to that of the H pures on hatching may be simply 

 a coincidence and not a case of dominance at all. 



In the case of the H hybrids the size on hatching is, as was previ- 

 ously stated, simply a physical necessity since the egg membrane 

 can contain an embryo of only a definite maximum size. 



The darker coloration of many H hybrids may also be explained 

 as a result of the blending of two characters. These individuals 

 combine the heavy-bodied type of chromatophore seen in H pure, 

 the closer aggregation of the latter (also a heteroclitus character), 

 with the darker pigment of the F. majalis parent. This combina- 

 tion gives the impression of a decidedly darker coloration. Of 

 course there are all degrees of blending with regard to the shape 

 and closeness of aggregation of the chromatophores, and in the 

 depth of color in the pigment, but those individuals that possess 

 the highest degree of similarity to the parent species in all three 

 respects give the impression of being more darkly colored than 

 either of the parent species. 



The case of hyperdominance involved m the comparative resist- 

 ances of hybrid and pure strains to lack of oxygen and the pres- 

 ence of carbon dioxide, may be explained as the physiological 

 result of the blending seen in the heart rhythms. The H hybrids, 

 being endowed with a more rapid and hence more efficient circu- 

 lation than the H pures, are enabled to withstand and throw off 

 the effects of the accumulating carbon dioxide in the system more 

 successfully than the H pures. On the other hand the M hybrids, 

 handicapped by a slower and less efficient circulation than the 



