FROM TWELVE TO THIRTY-SIX SOMITES 161 
in the form of a thick mass of cells in the roof of the neural tube. 
Towards the posterior end of the auditory epithelium the crest 
becomes smaller, and this is the beginning of the post-otic crest. 
Behind the ear the crest becomes larger again and extends later- 
ally so as to form a periaxial layer between the ectoderm and 
the axial mesoblast which extends back, above the first, second, 
and third somites to the middle of the fourth. The part between 
the ear and the first somite is, however, by far the best developed, 
the continuation behind being a relatively slight cord of cells. 
At about the stage of 17 somites the anterior part of the crest 
condenses to form a well-defined periaxial cord, which arises 
from the neural tube above the middle of the auditory pit, arches 
back behind its posterior margin and extends down into the 
third visceral arch, where it enlarges. This is the glossopharyn- 
geal periaxial cord. There is an enlarged portion of the crest 
just behind this overlying the site of the future fourth and fifth 
arches, but its substance is not yet condensed to form a distinct 
periaxial cord. 
At the stage of 20 somites the anterior cardinal vein and the 
duct of Cuvier form the posterior boundary of the enlarged por- 
tion of the post-otic crest (Fig. 73). The part of the periaxial 
layer immediately in front of this is somewhat condensed to 
form the periaxial cord of the vagus, and this is only indistinctly 
separated from that of the glossopharyngeus. 
The formation of the third visceral cleft definitely splits the 
periaxial layer into the periaxial cords of the glossopharyngeus 
and vagus (25s). This division is carried up indistinctly, at 
first, into the roots which occupy the space between the auditory 
sac and the first somite. The formation of the fourth visceral 
pouch similarly divides the distal portion of the vagus cord, 
so that part of it lies in front of the pouch and part behind. 
At the stage of seventy-two hours the ganghon petrosum 
glossopharyngeus) is definitely formed by an enlargement of 
the cord just above the third visceral arch, and the ganglion 
nodosum (vagus), similarly formed from the vagus cord, hes 
above the fourth visceral pouch, thus extending over the fourth 
and fifth arches. Branchial sense organs are formed at the dorsal 
angles of the second and third visceral furrows in connection 
with the IX and X nerves respectively. 
It would appear that the neural crest in the head is the 
