C26 H- H. ISl ewinau 



now noticeably larger than the M hybrids, were not nearly so 

 heavily pigmented. A certain degree of pigmentation had appeared 

 since the last stage described, but the color was still hardly visible 

 to the naked eye. The chromatophores of M pure were small- 

 bodied and much branched, quite different in appearance from 

 those seen on the H pures and the two types of hybrids. The 

 chromatophores of these three strains are practically alike. They 

 are much thicker bodied and less branched than those of M pure. 

 In other words, the type of chromatophore of F. heteroclitus 

 seems to be dominant. 



At II days: The H hybrid embryos, with the exception of 

 about 25 per cent which were ill-developed and obviously unhealthy, 

 showed heavier pigmentation than the average H pure. The 

 chief reason for this seemed to be that the pigment of the paternal 

 species (F. majalis) is blacker, even if more diffuse, and, when 

 aggregated in denser masses as it is in the hybrid, it gives a darker 

 coloration. The H hybrid embryos were on the average larger 

 than the H pures and the egg membranes seemed to be under 

 some tension. The M hybrids seemed to have stopped growing, 

 while the M pures had advanced rapidly. There was at that time 

 a marked disparity in size between the M pure and M hybrid 

 strains, the former being about 50 per cent larger than the latter. 

 A few unhealthy M pure embryos were noticed at this time. 



At 12 days: 24 H pures hatched out during the forenoon of this 

 day, before any change in the water had been made, and hence 

 without any artificial stimulus. A camera drawing of one of the 

 best of these was made on hatching, the specimen being quieted 

 with chloretone (Plate V, Fig. H). The figure shows a dorsal 

 view. During the first four hours of the afternoon twenty-five 

 more H pures hatched, along with two specimens of H hybrid. 

 In almost every particular these two hybrids resembled the H 

 pures that had just hatched. They were as darkly pigmented 

 as the darkest specimens of the H pures, were a little larger than 

 the average H pures, but not noticeably so. The color pattern 

 on the head and back was identical with that on the H pures. 

 These two hybrids were also as early to hatch as the average H 

 pures, and hatched without any artificial stimulus. The M hybrids 



