DIFFERENTIATIONS OF EOTODERM IN NECTURUS. 4985 
intersegments above this cleft has disappeared, like the dorso- 
lateral, in giving rise to facial mesectoderm. 
A distinct sensory area is found above the hyobranchial cleft, 
but posterior to this intersegment the epibranchial ridge is 
only interrupted where the region from which the sense-organs 
above the gill clefts are to be formed is separated from the 
club-shaped ridge that begins the main lateral line of the 
trunk. It will be noticed in fig. 1 that the posterior extension 
of the vagus ganglion lies between these two divisions of the 
epibranchial line. At an earlier stage this extension of the 
ganglion formed with the lateral line ridge and the ridge above 
the gill arches one deep ectodermic thickening. The two 
ridges here separated are consequently morphologically parts 
of the same ridge out of which a section has been cut to form 
the ganglion. 
Pl. 36, fig. 4, represents a cross-section though the region 
just described in a younger embryo than that of fig. 1. The 
plane of the section is indicated in fig. 1 by a corresponding 
number. In fig. 4 the large vagus ganglion is being cut from 
the primitive epibranchial ridge (p. ep. 7.), leaving a few deep 
cells above, which later increase in number and rearrange 
themselves to form the beginning of the lateral line of the 
trunk. Below the ganglion, cells are migrating from the 
ectoderm into the ectodermic connective tissue, but, although 
the whole ectoderm is here deep, that special elevation which 
constitutes the beginning of the sense-organs above the bran- 
chial arches has not yet appeared, the original ridge being 
temporarily obliterated in this region by the formation of 
mesectoderm. 
The most ventral of the three primitive longitudinal ridges 
(fig. 1) begins near the oral invagination, and extends backwards 
touching the ventral margin of the successive gill clefts. The 
ridge ends at the transverse ridge in the posterior margin of 
the last gill cleft. It therefore extends as far as the second 
intersegment posterior to the ear. I have previously called 
attention to the fact that the branchial clefts do not correspond 
to the intersegments, inasmuch as the second and third vagus 
VoL. 38, PART 4.—NEW SER. LL 
