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 
Fic. 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. ec. 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. 
