FATE OF NEURAL CREST IN HEAD OF URODELES . ral 
to the last pharyngeal pouch, the neural crest forms a broad 
sheet which will be perforated by the pharyngeal pocket of the 
third true gill. In this stage the neural-crest portion of the X 
ganglia possesses two ventral extensions or pharyngeal ganglia 
in addition to the third broad band lying behind the last pha- 
ryngeal pouch. 
In the lateral mesoderm posterior to the anterior end of the 
alimentary canal there is no change except that it is perforated 
by an additional pharyngeal pouch. The withdrawal of the 
lateral mesoderm in the region of the last pharyngeal pouch 
is more extensive than the contact between the ectoderm and 
endoderm, due apparently to the fact that this pouch excludes 
both ectodermal mesenchyme and entodermal mesenchyme in 
reaching the ectoderm. Mesoderm is certainly absent over a 
much larger area after the contact is formed than is ectodermal 
mesenchyme. Anterior to the anterior end of the alimentary 
canal the entodermal mesenchyme occupies the whole head 
region except the area ventral to a line drawn from the region 
of the epiphysis to the base of the hypophysis. In this region 
at this stage the mesenchyme is derived from ectoderm. ‘The 
area of overlap in the anterior head region is slight and can be 
best understood by reference to the reconstruction (fig. 11). 
In the posterior head region the ectodermal mesenchyme and 
mesoderm are coextensive except in the regions between the 
ganglia V, VII, IX, and X and in the region posterior to the 
X ganglia. ? 
The general result of the migration of the neural crest may 
be stated as follows: it furnishes first the general somatic and 
general visceral portions of the V, VII, IX, and X ganglia. 
Whether it furnishes other portions of the V, VII, IX, and X 
ganglia must be deferred to a later paper. It also migrates 
into and furnishes mesenchyme in the early stages in the mandib- 
ular, hyoid, and pharyngeal gill bars. The mesenchyme at 
first lies lateral to the endodermal derivative which is always 
more dense in the bars, but later comes to completely surround it. 
~The neural crest also furnishes a complete sheet of mesenchyme 
in the extreme ventral head region. The crest disappears as a 
