490 JULIA B. PLATT. 
this region, in preparation for the formation of the large vagus 
ganglia. 
I may mention here that while the hyobranchial and the 
first two vagus clefts arise in the line of a corresponding inter- 
segment, after the second vagus cleft has appeared the entire 
branchial apparatus begins to change its position in rela- 
tion to surrounding structures, and the second vagus cleft in 
consequence pushes forwards, so that when the third cleft 
appears it lies in the intersegment that was originally occupied 
by the second cleft. Hence the primitive somatic and branchial 
segmentation do not entirely correspond. 
The migration of cells from the ridges in the ectoderm, 
which I have described, is not by the wandering of cell after 
cell into the tissue below, but by the peeling or splitting off 
of the cells en masse, leaving the ectoderm above for the 
time no deeper than on the surrounding surface of the head. 
A new limiting membrane forms; and where we once found 
an area of deep ectoderm we now find a mass of mesectoderm 
cells, covered by ectoderm of no unusual depth. 
The primitive longitudinal and the transverse ridges of the 
trunk J described in my first study (82) as transitory differen- 
tiations of the ectoderm, which disappear and leave no clue to 
the cause of their existence unless interpreted in the light of 
later events, when it is found that the three lateral lines of 
sense-organs occupy the position once held by the primitive 
longitudinal ridges. 
The result of further study in regard to the primitive ridges 
of the trunk is recorded in this paper; the supposition, how- 
ever, that the ridges indicate sensory differentiations in the 
ectoderm, which are subsequently to be developed in the lateral 
line system, is not vitiated by their temporary disappearance, 
for ectodermic thickenings and ridges on the head of which 
many of the cells are undoubtedly sensory, inasmuch as they 
develop into ganglia, disappear completely for a time in giving 
rise to cells of the mesectoderm, then reappear as the Anlagen 
of lateral line organs, while other similar lines on the head 
give rise in part of their length to sense-organs, and in part 
