158 



THE DEVELOPMENT OF THE CHICK 



in the neural suture; it is separated from the mesenchyme in the 

 region of the fore-gut by a considerable space. (We shall call 

 the latter portion of mesenchyme the axial mesenchyme of the 

 head, to distinguish it from the mesenchyme derived from the 



neural crest, which later lies lat- 

 eral to it, and which may thus 

 be known as the periaxial layer.) 

 The crest may be followed ante- 

 riorly to the extreme tip of the 

 neural tube, and posteriorly to 

 the region of the anterior intesti- 

 nal portal, which lies at about the 

 transverse level of the future au- 

 ditory pit (cf. Fig. 91). In the 

 region of the mid-brain it spreads 

 out laterally until its peripheral 

 cells reach the axial mesenchyme. 



Goronowitsch divides the pre-otic 

 portion of the neural crest into pri- 

 mary and secondary ganglionic crests, 

 the post-otic portion being the terti- 

 ary crest. According to his account 

 there is a decided difference in time 

 of origin of the primary and second- 

 FiG. 91. - Diagram of the cephalic ary crests; the primary, involving the 

 neural crest of a chick of about region of fore- and mid-bram, aris- 

 12 s. (After Wilhelm His.) ing before the secondary which in- 



cludes the region of the trigeminus 

 and acustico-facialis. I have not, however, found such a difference in 

 my preparations. 



At the stage of 10 somites the cells of the pre-otic neural 

 crest have lost their connection with the neural tube. Behind 

 the optic vesicles they have spread out laterally between the 

 axial mesenchyme and the ectoderm, where they form a prac- 

 tically continuous periaxial layer, distinguishable from the axial 

 mesenchvme bv its greater densitv, and hence deeper stain; 

 but apparently mingling with it at the surface of contact. 



In the stages immediately following (10-20 s), the portions 

 of the periaxial layer lying above the mandil^ular and the hyoid 

 arches condense and thicken, and form strong cords extending 



