8 JACQUES LOEB 



between Menidia cf and Fundulus heteroclitus 9 . (See figs. 17 

 and 18.) Their eyes were poorlj^ developed, they had a tendency 

 to form Cyclopean eyes, the yolk was incompletely digested and 

 the embryo too small. The heart was formed and beat, but no 

 circulation was established. Pigment was formed (in the draw- 

 ing most of the black pigment cells on the yolk sac were not indi- 

 cated, since the drawing was intended for another purpose) . 

 The head is not bent against the body, and so on. In all respects 

 these sickly embryos of pure breed resemble the hybrids between 

 Fundulus heteroclitus 9 and Menidiacf. 



Since NaCN acts through a retardation in the rate of oxidation, 

 the idea might be expressed, that in heterogeneous hybrids oxida- 

 tions are interfered with. It is not safe to accept such an idea 

 until it has been tested experimentally. 



6. The idea that the heterogeneous hybrids in fish are purely 

 or at least essentially maternal finds support in the fact that a 

 small number of these hybrids develop more normally than those 

 thus far mentioned. In a small number of such hybrids circula- 

 tion is established, though as a rule rather late and often for a few 

 days only. But these embryos develop rather normally and are, 

 as far as any present observations go, purely maternal. I have 

 had many such hybrids but I will give only one camera drawing 

 (fig. 19), representing a twenty-five day old hybrid between a male 

 scup and a female Fundulus heteroclitus. The yolk is in this case 

 pretty well digested. 



7. While it is the rule that in the case of heterogeneous hybrid- 

 ization heredity is purely maternal, it is possibly not without 

 exception. I have however thus far found only one paternal 

 character which is possibly transmitted to a heterogeneous hybrid. 

 The yolk sac of the egg of Fundulus heteroclitus forms branched 

 red chromatophores which are not found on the yolk sac of Meni- 

 dia. In two eggs of Menidia fertilized by sperm of Fundulus 

 heteroclitus a few red chromatophores were observed. It is diffi- 

 cult to get this cross and I give this observation with some 

 hesitancy. 



8. Kupelwieser and Baltzer account for the fact that hetero- 

 geneous hybrids are purely maternal by the assumption that the 



