246 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Branchial sense organ . — After its separation from tlie ear the branchial sense organ i 
has the character of a sac, one wall of which is much thicker than the other, and over|[ 
the cavity of which passes the epidermic stratum. Its appearance in surface view is ' 
given in Fig. 148, B. s. o., and a transverse section through a corresponding stage in 
Fig. 96, PI. xcix. Anteriorly and posteriorh^ as may be seen in the surface view, 
the sac is not sharply delimited. The further development of the organ consists in'^j 
the loss of its cavity, in histological differentiation, and in the transformation of its 
ill-defined anterior extremity into two cellular cords which doubtless serve as a source ' 
for the production of new organs. 
On comparing Fig. 96, PI, xoix, with a slightly older stage. Fig. 106, PI. c, 
it is plain that the cavity of the sac has grown shallower, and with this change the 
epidermic stratum has dipped into the cavity, which it now lines (Fig. 106). Thel 
effacementof the cavity, begun in this way, is continued until the originally conspicu- i: 
ous cavity is reduced to an extremely shallow indentation (Fig, 115, PI. ci), and at ; 
the time of hatching (Fig. 131, PI. civ, B. s. o.) even this indentation seems to have 8 
disappeared. " 
Coincidently with the loss of its cavity the sense organ changes its shape, as may,) 
be seen by glancing through the sections. Figs. 96, PI. xcix, 106, PI. c, 115, Pl.| 
CI, 131, PI. CIV, and the surface views, Figs. 148, 149 and 150. Whereas in the earlier I 
stages it is a vaguely delimited sac, in the later stages it is a shi^rply defined superficial !» 
sense patch, of an oval shape, from which there runs forward a short sensory cord, i 
a. s. t., Figs. 149 and 150. Histological modification accompanies the change of shape, i 
In Figs, 96 and 106 the cells are embryonic columnar cells, but in Figs. 115 and 131 the 
organ is largely composed of peculiar sense cells, the nuclei of which are basal. Some • 
of these cells terminate in short stiff hairs which project from the surface. No macer- > 
ations were made, and I am consequently not able to go any farther into details of | 
the histology, which appears to be identical with that of the sense organs of the lateral I 
line. It is plain that the organ is functional during the later stages of embryonic and ' 
during larval life. Its iiosition directly in front of the embryonic gill slit is shown in 
the surface views. 
When the sense organ assumes its definite shape, there is left in front of it a narrow 
tract of columnar cells in iierfect continuity with the sense organ and obviously derived ■ 
from the anterior part of the anlage shown in Fig. 148. This tract of cells is conspicuous ■ 
in surface views of some stages (Fig. 149, a. s. #.), but towards the time of hatching it ; 
becomes difficult to see in such views, though sections show that it not only persists 
but continues to develop. The entire anterior extension of the sense organ may be 
spoken of as the forward sensory tract. This tract differentiates into two tracts, one 
of which runs directly forwards (anterior sensory tract) while the other runs some- i 
what dorsally (dorso lateral tract). 
Figs, 106 and 107, PI. c, belong to the same series of sections and are from 
a stage intermediate between Figs. 148 and 149. Fig. 107 is through the anterior 
part of the forward sensory tract where it differentiates into the anterior tract {a. s. 
t.) and dorso lateral tract (d. 1. s. t.). Figs. 115 and 116, PI. ci, are from another 
series and belong to a stage about like Fig. 149. Fig. 116 shows the connection of 
the two sensory tracts at the anterior end of the whole forward extension. Near the 
time of hatching the dorso lateral tract extends a short distance in front of the ante- 
rior tract, occupying the position shown in Fig. 123, PI. Oil, d. s. t. During larval 
