DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 495 
Fig. 2,0, is a cross-section through the same region as fig.2,a, 
but in a younger embryo, in which as yet but one vagus cleft 
is found. The section shows the primitive ectodermic ridge 
connected with the trigeminal ganglion and ramus ophthal- 
micus profundus. Comparing the two sections a and 6, one 
sees that in 4 the ectodermic ridge has been produced by 
increase in the cell layers of which the ectoderm is composed, 
with no tendency to radial arrangement; while in section a the 
ridge is produced by the radial arrangement and depth of a 
group of cells in the lower of the two layers here composing 
the ectoderm. 
Pl. 36, figs. 3 and 4, are both from an embryo with two 
vagus clefts, and pass through planes given in fig. 1. But few 
cell outlines are drawn, since they are rendered indistinct by 
yolk granules, which I have omitted as unimportant in the 
present study. The plane of fig. 3 lies anterior to the hyoman- 
dibular cleft, and at the beginning of the facial ganglion, a few 
cells of which touch the ectoderm dorsally in the section, while 
below a few connective-tissue cells of ectodermic origin sepa- 
rate the ectodermic ridge from the wall of the alimentary 
canal. Only those mesectoderm cells which come in contact 
with the ridge are reproduced. The section shows a primitive 
ridge about to add its cells to the facial mesectoderm. In the 
dorsal part of the ridge may be seen a slightly radial arrange- 
ment of the cells. This appearance might indicate either the 
formation of a secondary sensory ridge from the primitive 
ectodermic thickening, or that a round mass of ganglion cells 
is about to be cut from the skin. Which of the two processes 
is actually taking place is to be discovered by comparing the 
next stage of development with the present. Such a com- 
parison shows that the section lies posterior to the branchial 
sense-organ above the hyomandibular cleft, and illustrates the 
initial stage in the excision of the epibranchial part of the 
facial ganglion. Fig. 4, which cuts the primitive ridge that has 
just given rise to part of the vagus ganglion, shows a similar 
excision at the point of completion. 
The three sections (figs. 2, 3, 4) give the relative depth of 
