FROM TWELVE TO THIRTY-SIX SOMITES 



159 



from the superior angles of the neural tube into the arches in 

 question; here they form connections with the ectoderm of the 

 arches, which proliferates so as to contribute to their substance 

 (Fig. 92). Elsewhere the periaxial layer gradually merges with 

 the axial mesenchyme. The periaxial cords are the primordia 

 of the trigeminus and acustico-facialis ganglia, and mark the 

 paths of the trigeminal and facial nerves. Their connection with 



FIG. 92. Transverse section immediately be- 

 hind the first visceral pouch of a chick 

 embryo of thirteen somites. (After Gorono- 

 witsch.) Note connection of the periaxial 

 cord with the ectoderm of the visceral arch. 

 Ad., Aorta descendens. c. Rounded me- 

 senchyme cells, g. Place where cells derived 

 from neural crest unite with the mesenchyme 

 cells of the periaxial cord. f. Fusion, p. Spin- 

 dle-shaped peripheral mesenchyme cells. 



the ectoderm in the neighborhood of the first visceral pouch 

 must not be confused with the so-called branchial sense-organs, 

 for the primary connection is soon lost, and secondary connec- 

 tions arise at about the 27 s stage, and constitute the true branchial 

 sense-organs of these arches. 



