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 (n) is cut through 

 neither anterior nor posterior opening, while on the 

 left side the canal shows the anterior opening (an). 



Figure 2O/, 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 (nt), with 

 the faintly outlined condensations of mesoblast in 

 their region, need no special description. The 

 pharynx (ph) 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 (la), 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 (vii) , above which is the 

 bulbus (b) and the tip of the auricle (au). The 



