HEREDITY OF PIGMENTATION 161 
In both hybrids these head chromatophores appear in essen- 
tially the same way that they do in the pure F. heteroclitus; 
there are, however, minor differences. In the hybrids they appear 
a little later, and they remain for a day or more on the sides of 
the brain without migrating onto its dorsum instead of only for 
several hours as in the case of the pure F. heteroclitus. Even 
after the chromatophores of the hybrids have reached the dorsum 
of the brain they do not look like any stage in the development of 
the same cells in the pure F. heteroclitus, for they are smaller 
and there are more of them. The comparative numbers of these 
cells for ten fish each of the pure F. heteroclitus and the F. heter- 
oclitus egg hybrid are given in table 2. The differences while 
not great are, I think, significant. In the F. majalis egg hybrid 
the size and numbers of these cells are essentially similar. 
TABLE 2 
° NUMBERS OF HEAD CHROMATOPHORES AVERAGE 
Pigeelm heserocktus fon mid-brain..| 2 | 0 | 0 | 1 | 5) 1) 2 | 0 | 0 | AN ssl 
, \on hind-brain. ein eae: | 4/413 } 2/0) 5 3.4 
| | | | | | 
¥. heteroclitus egg {on mid-brain..|6|6|5/1)3|0/6|5 [10 | 4 4.6 
hybrid......... (on hind-brain.| 5 | 4 | 2/3 | 4.7 | 4 | 6|3/4 4.2 
In the pure F. majalis, on the other hand, there was no sign of 
this first crop of head chromatophores except in one case, out of 
several hundred embryos examined; and in this case the first crop 
was represented only by a single cell. With this single exception 
the first chromatophores to appear on the head of F. mayalis 
were similar in all respects to the second crop of the other forms. 
The second crop of head chromatophores in the pure F. heter- 
oclitus and the two hybrids always appeared at a distinct interval 
after the first crop. This interval varied from one to four days. 
In the hybrids this crop usually appeared one or more days later 
than in the pure F. heteroclitus; but in the pure F. majalis it 
appeared at the same time as in the other pure form. In all four 
forms the method of development of this crop was the same, and 
was different from that of the first crop. As far as could be seen 
