158 FRANK W. BANCROFT 
2. Black yolk chromatophores 
For the first few days of their existence the black yolk chroma- 
tophores of both hybrids were identical with those of the pure 
F. heteroclitus in shape and the differences in size could appar- 
ently be entirely accounted for by differences in the ages of the 
cells. Compare figs. 18 and 14, 18 and 19 for the resemblance 
between the pure F’. heteroclitus and the F. heterolcitus egg hybrid 
and figs. 8 and 11 for the resemblance between the pure F’. heter- 
oclitus and the F. majalis egg hybrid. In the pure F. majalis 
these cells were on the whole smaller and with more and longer 
processes than in the other three forms (see figs. 10, 12). This 
difference in F. majalis was most pronounced in those cells which 
were nearest to the embryo, as in fig. 10; the cells that were far- 
ther from the embryo (fig. 12) differed less in shape and size 
from those of F. heteroclitus. 
We have seen that for the early stages there was complete dom- 
inance of the F. heteroclitus condition, as has been stated by 
Newman.‘ With further development, however, characteristic 
differences appeared so that each form could be distinguished 
by the appearance of the black yolk chromatophores alone. 
In the pure F. heteroclitus these chromatophores soon crept 
onto the blood vessels, forming a coarse reticulum in which the 
clear spaces between the chromataphores were in general equal to or 
wider than the space occupied by the chromatophores themselves. 
There were very few branches traversing these clear spaces. 
In the F. heteroclitus egg hybrid the chromatophores did not 
hug the vessels so closely so that the width of the colored meshes : 
was usually about twice the thickness of the clear spaces between 
them. These clear spaces were also usually thickly traversed by 
a feltwork of fine processes (fig. 23). The color of this reticulum 
was also paler than in the pure F. heteroclitus, apparently because 
the chromatophores were spread over a greater area. 
In the F. majalis egg hybrid there is, on account of the greater 
size of the egg, agreater space to be covered by thechromato- 
phores. Together with this factor there was a much less pro- 
4 Jour. Exp. Zool., vol. 5, p. 550. 
