166 FRANK W. BANCROFT 
7. Rate of development of yolk chromatophores 
In the pure F. heteroclitus the black yolk chromatophores first 
appeared when the heart was beginning to beat and before a cir- 
culation had been established; also before the fore-brain had 
acquired any lumen. The embryo had about twelve somites. 
The red chromatophores could usually not be seen until the next 
day. 
In the F. heteroclitus egg hybrids the black yolk chromato- 
phores also appeared at the time when the heart was first beating 
and before the circulation had started. In this form, however, 
the heart-beat and circulation started a little later than in the pure 
F. heteroclitus. At this time the fore-brain of the embyro 
usually had something of a lumen (condition was intermediate 
between figs. 1 and 2). Accordingly in this form the first appear- 
ance of the yolk chromatophores was later in time and also at a 
later stage in the development of the embryo. 
In the F. majalis egg hybrids the yolk chromatophores did not 
appear until after the circulation was established, and until the 
embryo had a large lumen in the fore-brain like fig. 4. 
In the pure F. majalis the yolk chromatophores appeared twelve 
to twenty-four hours later than in the F. majalis egg hybrids, at 
a time when the embryo was in a stage about half way between 
those represented in figs. 4 and 5. 
Thus it is seen that both with respect to the time, and with 
respect to the development of the embryo, the hybrids had rates 
of development intermediate between those of their parent forms, 
and there was no indication of Mendelian dominance. The dis- 
covery, however, of factors which necessitated a Mendelian inter- 
pretation of the development of the head pigmentation, which, 
at first sight appeared exactly similar to this case of yolk pigmen- 
tation, makes one suspect that more study may result also in a 
Mendelian interpretation of the rate of development of the yolk 
chromatophores. 
Although in this case the intermediate position of the hybrids 
and the lack of dominance is most evident, the same phenomenon 
is seen to a less degree in the development of the black chroma- 
