DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 487 
namely, of that ridge which marks the separation of the proto- 
vertebra from the nephrotome. The most ventral of the ridges 
on the trunk marks the line at which the nephrotome separates 
from the remaining parietal plate. This line is continued later 
on the head by a ridge which, beginning at the oral invagina- 
tion, extends backwards below the gill clefts as these succes- 
sively form, and finally unites with the ventral ridge of the 
trunk. 
As the protovertebree are cut off from one another, the 
median longitudinal ridge becomes differentiated by a series of 
intersegmental elevations which correspond to intersegmental 
expansions of the alimentary canal. ‘This appears to favour 
Boveri’s (5) theory that gill clefts once extended throughout 
the length of the body, for in the branchial region these inter- 
segmental elevations on the median longitudinal ridge mark the 
beginning and the dorsal limit of the gill clefts, while on the 
trunk of the embryo they extend towards, although they do not 
touch, the corresponding intersegmental expansions of the 
alimentary canal. 
The longitudinal ridges soon become connected with one 
another by transverse intersegmental ridges. On the head of 
the embryo these transverse ridges are formed from the separate 
elements of the line of dorso-lateral proliferation of mesecto- 
derm, which severally unite with a corresponding epibranchial 
thickening of the ectoderm, which, in turn, blends with the 
upper limit of that transverse ridge of ectoderm which meets 
the endoderm prior to the formation of the gill clefts. 
If the skin of an embryo at this stage be removed and 
viewed by transmitted light, it is seen to be divided into a series 
of small squares, each of which is bounded by the longitudinal 
and transverse ridges I have just described, and each of which 
corresponds to the outer surface of a protovertebra. This 
method of determining the position of the ectodermic ridges 
cannot be applied in the branchial region or in the anterior 
part of the head, since the rapid proliferation of mesectoderm, 
which is here taking place, renders it for the time impossible 
to separate the ectoderm accurately from the tissues beneath. 
