. The development of the chick; an introduction to embryology. Birds -- Embryology. 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-
. The development of the chick; an introduction to embryology. Birds -- Embryology. 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 cord
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