162 FRANK W. BANCROFT 
these cells developed in situ; appearing first as faint, grey, thin 
and well branched cells scattered all over the dorsum of the brain. 
Soon after their first appearance their color became much deeper 
and they began to expand rapidly, so that after several days it 
was no longer possible to distinguish them from the chromato- 
phores of the first crop. Fig. 6 shows the appearance of this 
second crop of head chromatophores in the same embryo which is 
represented nineteen hours earlier in fig. 5. A little later stage 
after the chromatophores of the second crop have become so 
large that they cannot always be certainly distinguished from 
those of the first crop is shown for the F. majalis egg hybrid of 
series 7 in fig. 9. To be compared with this last figure is fig. 10 
taken from an embryo of the pure F. majalis also of series 7 and 
having the same age as the embryo figured in fig. 9. The entire 
absence of anything at all corresponding to the first crop of 
chromatophores is very evident. 
It is very clear then that the F. heteroclitus character ‘presence 
of the first crop of head chromatophores’ dominates over the F. 
majalis character ‘absence of this crop.’ Furthermore, since the 
second crop of chromatophores appears in F. majalis synchronous- 
ly with its appearance in F. heteroclitus, the delayed appearance 
of both crops in the hybrids cannot be considered a condition 
intermediate between that of the two pure forms. 
4. Red chromatophores of the lateral line 
In the pure F. heteroclitus and the F. heteroclitus egg hybrids 
at the time of hatching and the F. majalis egg hybrids (which 
usually do not hatch) at about the same time the lateral line is 
mapped out by a series of about twenty conspicuous red chroma- 
tophores the characteristics of which are shown in figure 27. In 
all of these three forms these cells were very similar in appear- 
ance, and there can be no doubt that they are essentially the 
same kind of cells as the red yolk chromatophores. In the pure 
I. majalis these red chromatophores were not present on the 
lateral line at hatching time. There was, however, aseries of very 
