164 FRANK W. BANCROFT 
5. Black chromatophores of the lateral line 
The pure F. majalis upon hatching and shortly before hatch- 
ing had a series of from 40 to 60 black chromatophores along the 
lateral line (fig. 25). There were usually two chromatophores to 
the segment. One of these was near the surface of the fish and 
expanded in a plane parallel to the surface. The other was situ- 
ated farther from the surface upon the septum in the frontal 
plane separating the dorsal from the ventral musculature. This 
second chromatophore expanded in the plane of this septum. 
The pure F. heteroclitus at hatching time and before, usually 
had no black chromatophores at all along the lateral line. Per- 
haps ten per cent had one or two black chromatophores along the 
lateral line, and a much smaller percentage had more. But none 
were seen which had more than ten or twelve black chromato- 
phores in this place. Upon the first day after hatching, however, 
80 per cent or 90 per cent of the fish were found to have black . 
chromatophores varying in number from 1 to 26 and averaging 
about 8. During the next few days this increase continued until 
all the fish had from 20 to 30 black chromatophores along the 
lateral line. 
The F. heteroclitus egg hybrids were on the whole similar to 
the pure F. heteroclitus, but exhibit a slightly intermediate con- 
dition as the black chromatophores begin to appear on the lateral 
line a little earlier than in the pure F. heteroclitus. Thus at 
hatching time the hybrids usually had enough chromatophores 
so that above the anus, when expanded, they made a continuous 
line of black, with scattered black cells posterior to this region. 
Fig. 26 represents part of a fish of this kind, in which, however, 
the chloretone given to quiet the animal has caused the chroma- 
tophores to contract slightly and thus break up the continuous 
black line which was originally present. In the same series a 
comparison of the lateral lines a few days after shows that at 
this time also the hybrids maintained their lead in the develop- 
ment of the black chromatophores along the lateral line. At that 
time ten of the hybrids averaged 29.2 black chromatophores to a 
