194 Henry Fox 



portion is very deep. The deepest part is represented by the blunt 

 angle immediately below the lower end of the lateral border. 



Posterior to the region of the second pouch the pharynx diminishes 

 considerably in width. Its lateral margin forms a low ridge connecting 

 the second pouch with the third. Between this ridge and the median 

 dorsal ridge of the pharynx is a shallow longitudinal furrow, in which 

 is lodged the dorsal aorta. 



The third pharyngeal pouch is slightly smaller than the second. It 

 is joined by a relatively narrow base with the pharynx, but distally 

 expands into a broad wing-like fold with a prominent ventral divertic- 

 ulum. A slight dorsal diverticulum is also present. The lateral margin 

 is in contact with the ectoderm for almost its entire length. 



As in the case of the second pouch, the deep portion of the ventral 

 diverticulum is limited to the lateral half of the pharynx. Its mesial 

 portion is represented by a low ridge, which extends from the root of 

 the lateral half forwards and inwards close to the median line, where 

 it joins with the same part of the second pouch. The extreme ventral 

 tip of the ventral diverticulum is turned toward the mesial side. 



The fourth pharyngeal pouch is the smallest of the series. It is 

 divided by a shallow constriction into two parts, a dorso-posterior por- 

 tion (Ph. P. IV), which projects laterally and at one point comes into 

 contact with the ectoderm, and a ventro-anterior bulge, which terminates 

 blindly and corresponds to a ventral diverticulum. 



From the base of the ventral diverticulum a low ridge extends for- 

 wards to the base of the third pouch. It corresponds to the mesial 

 extension of the ventral diverticulum. 



In the cat embryo of 6.22 mm. (No. 480, Harvard collection) the 

 condition of the pharynx is essentially similar to that just described in 

 the pig. As in the latter, four pairs of pharyngeal pouches are present. 



The characteristic features of this stage are shown in Figs. 58, 59 

 and 60. Fig. 58 shows the lateral aspect. The clear areas on the 

 lateral margins of the pouches show the extent to which these are 

 attached to the ectoderm. It will be noticed that they are essentially 

 similar to the same parts in the preceding specimen. The dorsal 

 diverticulum of the first pouch is somewhat more elevated. The ventral 

 diverticulum of the third pouch extends to a slightly lower, level. The 

 fourth pouch shows more clearly its division into two portions. The 

 dorso-posterior portion is somewhat bulbous. Its more dorsal part is 



