The Process of Heredity 549 



apparent Inability to consume the large mass of superfluous 

 yolk. 



7 The heart-beat of the H pures appears about ten hoursearlier 

 than that of the H hybrid and this gives the former a decided 

 advantage over the latter in rate of subsequent development. 

 When, however, the H hybrids attain a heart-beat and a circulation 

 it is a more rapid and efficient one than that of the H pures (the 

 more rapid heart-beat being an endowment from the paternal 

 species), and they rapidly overhaul the H pures and, for a time, 

 seem to be almost on an even footing with the latter, a small per- 

 centage of them certainly surpassing any of the H pures. The 

 heart-beat and circulation of M hybrids appears nearly a day 

 earlier than that of the M pures, and for a time the former show 

 a decided advantage over the latter. But when the M pures 

 attain a heart-beat and circulation it is a more rapid and efficient 

 one and the M pures overtake and pass the M hybrids. The 

 latter remain behind permanently and, with their slower heart- 

 beat and less efficient circulation they never succeed in consuming 

 more than half of the yolk with which the egg is endowed. 



8 The phenomenon of pigmentation runs parallel with that of 

 circulation and is probably dependent to some extent thereon. 

 The H pure embryos become heavily pigmented in about three 

 days, while the M pures become pigmented very late in develop- 

 ment, and then comparatively lightly. Naturally the H pures 

 show the first signs of pigmentation, dark brown chromatophores 

 appearing on yolk and body soon after the establishment of a cir- 

 culation. About a day later the H hybrids begin to show signs 

 of pigmentation, but at first far less abundantly than the H pures. 

 In later stages, however, the most successful of the H hybrids 

 surpass in depth of pigmentation the most heavily pigmented of 

 the H pures. This may be explained by the fact that these hybrids 

 combine the F. heteroclitus character of densely packed chroma- 

 tophores with the F. majalis character of a darker colored pig- 

 ment. The M hybrids show a much earlier and much heavier 

 pigmentation than do the M pures. The difference at about five 

 days is very evident to the naked eye. The color pattern on the 

 head and trunk, characteristic of F. heteroclitus embryos, is 



