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 ganglion 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, lies 

 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 



