DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 489 
of the epibranchial ridge. It would be possible to associate 
that proliferation of cells to the ophthalmicus profundus, 
which constitutes the ciliary ganglion with the lens as dorso- 
lateral and epibranchial differentiations in the intersegment 
between the mandibular and premandibular head-cavities. 
These cavities are not distinct in Necturus. 
My description of the changes that take place in the ectoderm 
prior to the stage with which this study begins may be more 
easily followed by referring to the figures given with the first 
study (82). 
Between the two somites above the hyomandibular cleft we 
find at an early stage two circular areas, in which the ectoderm 
becomes deep in both the dorso-lateral and epibranchial lines. 
The two areas unite with one another later in an intersegmental 
ridge, which meets the intersegmental proliferation of cells to 
the trigeminal mesectoderm above the hyomandibular cleft. 
Thus the hyomandibular cleft is the ventral continuation of 
two intersegmental lines, one connected with the trigeminus, 
the other with the facialis. The specialised areas connected 
with the facialis increase greatly in depth and extent, and at 
the time when the migration of cells to the facial mesectoderm 
is most rapid the dorsal thickening becomes continuous, on 
the one hand with the auditory epithelium, and on the other 
with the trigeminal portion of the dorso-lateral ridge. 
The specialised areas of ectoderm in the next intersegment 
are the ear on the dorso-lateral line, and the epibranchial 
thickening above the hyobranchial cleft. The two areas 
become sharply separated from one another only as the ear 
becomes constricted off as a closed vesicle. From the auditory 
epithelium cells migrate into the auditory ganglion, and from 
the epibranchial thickening into both facial and glosso- 
pharyngeal mesectoderm. 
The two following intersegmental ridges are above the first 
two branchial clefts, and are connected with the migration of 
cells into the vagus mesectoderm. In these intersegments it 
is not easy to distinguish the dorso-lateral from the epibranchial 
thickening of the ectoderm, so deep is the entire ectoderm in 
