DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 527 
and mandibular lines is considerable; and without the inter- 
mediate stage given in fig. 21, one would hardly identify these 
lines with the primitive ridges seen in figs. 1 and 18, from 
which they have developed. The changes, however, are chiefly 
connected with the forward growth of the mandible and the 
conversion of a head originally deeper than wide into a broad 
flat head. The dorsal part of the hyomandibular line has 
given rise to a double row of sense-organs; and although the 
line was originally nearly vertical, the dorsal part of the line 
is now horizontal, the change of direction begun in fig, 21 
having further increased. The anterior (once dorsal) extremity 
of the line, moreover, now meets the infra-orbital line back of 
the eye. 
The mandibular line, which originally made an obtuse angle 
with the dorsal part of the hyomandibular line, now leaves 
that line at an acute angle, and at the corner of the mouth 
closely approaches the base of the hyomandibular line. Here 
one or two sense-organs lie between the mandibular and infra- 
orbital lines, but anterior to this point no sense-organs, save 
those of the supra- and infra-orbital lines, are found on the 
upper lip. The mandibular line then bends on to the lower 
lip, near the middle point of which it ends in close proximity 
to the mandibular line of the opposite side. The two lines are 
distinct, however. 
Although the median lines of sense-organs on the lower jaw 
do not primarily belong to the hyomandibular ridge, but to 
the ventral longitudinal, as the organs are innervated by a 
nerve that directly continues the main hyomandibularis, I 
prefer to speak of them as the anterior part of the hyoman- 
dibular line rather than as independent lines. PI. 38, fig. 33, 
shows that each of these two median lines has now given 
rise to a double row of sense-organs, and although they are 
no longer united at their anterior extremity with the outer, 
mandibular, lines, as an index of the former union, seen in 
fig. 21, the innervation of the inner row of sense-organs at the 
auterior extremity of the mandibular line comes from the 
nerve supplying the hyomandibular line (fig. 31). From 
