The Development of the Alligator 323 
The auditory vesicle (0) on the right is cut near its 
middle region, while that on the left is barely 
touched by the plane of the section. The noto- 
chord (nt), with its condensed area of mesoblast, 
is somewhat larger than in the preceding section. 
The nasal canal on the right (x) is cut through 
neither anterior nor posterior opening, while on the 
left side the canal shows the anterior opening(a). 
Figure 20f, which is in the region of the posterior 
part of the pharynx and the anterior part of the 
heart, shows some rather unusual conditions. 
The spinal cord (sc) and notochord (u#), with 
the faintly outlined condensations of mesoblast in 
their region, need no special description. The 
pharynx (p/) is here reduced to an irregular, trans- 
versely elongated cavity, the lateral angles of 
which are connected on each side with the exterior 
through a tortuous and almost closed gill cleft 
(g), which must be followed through many sections 
before its inner and outer openings may be deter- 
mined. Dorsal to the pharynx numerous blood- 
vessels (bv), both large and small, may be seen, 
while ventral to it is noticed a faint condensation of 
mesoblast (Ja), in the form of an inverted T, the 
anlage of the laryngeal structures. The ventral 
portion of the figure is made up of a nearly circular 
thin-walled cavity, the pericardium (pr). Most of 
the pericardial cavity is occupied in this section by 
the thick-walled ventricle (vn), above which is the 
bulbus (0) and the tip of the auricle (aw). The 
