HEREDITY OF PIGMENTATION 159 
nounced tropism of the chromatophores for the blood vessels, 
so that hereno characteristic chromatophore reticulum was formed. 
In addition there was much variation in the shape of the chroma- 
tophores during these later stages; some of them still retained 
their previous characteristic polygonal shape, while others devel- 
oped long processes like the red yolk chromatophores or the black 
yolk chromatophores in the pure F. mayjalis. 
In pure F. majalis the black yolk chromatophores were charac- 
terized by their long and numerous processes also during these 
later stages. 
In considering these later deviations from the complete domi- 
nance of the early stages it might be thought that we had here two 
intermediate conditions manifesting themselves in the hybrids 
due to a delayed influence of some ‘branching factor’ derived from 
F. majalis. For in both cases the deviations are in the direction of 
the more profuse branching characteristic of F. majalis. I think, 
however, that in this case the deviations from complete dom- 
inance are due mainly to a diminution of the tropism of the chro- 
matophores for the blood vessels discovered by Loeb.> This 
reduced chemotropism is probably due to a change in the contents 
of the blood vessels. resulting from the diminished yolk assimi- 
lation in the hybrids which will be discussed later. However, no 
matter what its cause, there can be no doubt that in both the 
hybrids the yolk chromatophores are less closely approximated to 
the blood vessels than in either of the pure forms. Now when in 
5 Jour. Morph. 1898, viii, 161; Arch. f. d. ges. Physiol. liv, 525. Loeb at first 
thought that this tropism was chemotropism due principally to oxygen. Later 
he was inclined to consider that it might be stereotropism. Among the embryos 
in this series it was noticed in a number of cases that after the circulation had be- 
come well established and the chromatophore network was well formed that the 
blood accumulated in some one region and stopped circulating through the ves- 
sels, although the heart still continued beating normally for several days. When 
this stoppage of the circulation took place in a comparatively early stage it was 
seen on several occasions that the chromatophores left the vessels and became 
again uniformly distributed over the surface of the yolk. As the vessels were 
still present it must be concluded that chemotropism and not stereotropism is 
responsible for the creeping of the chromatophores on to the blood vessels. When 
the circulation stopped in later embryonal life the chromatophores usually con- 
tinued to remain on the blood vessels even several days after the blood had ceased 
to circulate. 
