550 H . H. Nezvrnan 



found on all embryos of the M hybrid at the period of their maxi- 

 mum development. The same pattern is found on all successful 

 H hybrids embryos in advanced stages. 



9 The chromatophores of F. majalis are small-bodied and 

 finely branched, while those of F. heteroclitus are proportionally 

 larger bodied and much less finely branched. The F. heterocli- 

 tus type of chromatophore is found in three out of the four strains, 

 viz: H pure, H hybrid, and M hybrid. 



10 The size of F. majalis on hatching is about twice that of 

 F. heteroclitus. That o{ H hybrids is, on hatching, no larger, on 

 the average, than that of // pures. The maximum size reached by 

 the M hybrids is about equal to that of the H pures on hatching. 



1 1 The examination of preserved material furnished more 

 easily measurable data for demonstrating the differences at vari- 

 ous stages of development between pure and hybrid strains. The 

 numbers of mesoblastic somites at various stages offers an accurate 

 and convenient measure of stages of development. In every case 

 examined the data derived from preserved material served to con- 

 firm observations made upon living material. 



12 The hybrid strains, especially the H hybrids, showed a 

 remarkable degree of variability, far wider in range than was seen 

 in the pure strains. 



13 In the experiments involving the lack of oxygen and the 

 presence of carbon dioxide, the two hybrid strains, instead of 

 occupying positions between the two pure strains, fell to the 

 extremes. The H hybrids were the most resistant, the H pures 

 next, the M pures next, and the M hybrids the least resistant. 



14 KCN solutions in sea-water were found to act in quite a 

 different way from carbon dioxide or lack of oxygen. Here the 

 hybrid strains seemed to occupy mean positions, although with 

 no great degree of regularity. The data derived from the use of 

 KCN was not very satisfactory. 



