518 JULIA.E. PLATT. 
changed, but has annexed to itself, as the innervation shows, 
the anterior part of the ventral longitudinal line. That part 
of the original ventral ridge, which bounded the branchial 
clefts ventrally, has now disappeared, and the ridge can be 
followed but little beyond the plane of the hyomandibular 
ridge. 
The glossopharyngeal ridge has divided into two sensory 
areas. ‘The long axis of the upper, like that of the undivided 
ridge, is vertical, while the long axis of the lower division is 
now longitudinal. In the interval between the two areas the 
deep sensory cells of the original ridge have entirely dis- 
appeared from the ectoderm. 
The posterior intersegmental ridge is little changed. The 
nerve which supplies it appears at this stage of development 
to enter the vagus ganglion by a root distinct from that of the 
nerve supplying the epibranchial or horizontal part of the 
ridge. The two nerves, however, enter the ganglion by a 
common root in a slightly older embryo. 
The ridge at the posterior margin of the last branchial cleft 
no longer exists. The slight ventral extension of the ridge 
seen in fig. 18 has grown backwards as the ventral trunk line 
which is now about to pass beneath the arm, and has conse- 
quently reached the region in which the anterior sense-organs 
of this line are to form. No trace of the ventral ridge is 
found in the space through which it has just passed from the 
posterior margin of the gill cleft to its present position. The 
anterior cells of this ridge have migrated as did those of the 
dorsal and median trunk ridges. 
Four distinct organs lie in the path of the dorsal line, and, 
as fig. 21 shows, their position is not strictly segmental. There 
is, however, a strong tendency in the dorsal trunk line to form 
a sense-organ at each intersegment, and the anterior organ of 
the line most frequently lies between the third and fourth 
myotomes (no account being taken of the division of the vagus 
myotome, the anterior part of which is rudimentary). In the 
embryo represented in fig. 21 the anterior sense-organ of the 
dorsal trunk line lies above the fourth myotome. 
