198 Henry Fox 



degree the remainder of the pharynx, tending to displace it to a lower 

 level. This tendency, however, is checked by the fact that the first pair 

 of pouches is still attached to the ectoderm l)y their lateral extremities. 

 These points are accordingly relatively fixed in position, and, as the 

 basal portion of the pouch sinks in response to the general lowering of 

 the pharynx, the structure attains the pronounced ascending course 

 characteristic of it at this stage. 



In consequence of this change the basal portion of the pouch has 

 assumed an almost horizontal plane, while its peripheral part ascends 

 almost vertically. Where the two parts meet there is on the lateral 

 surface a slight ridge extending from the lateral ridge to the base of 

 the vestibular fold. It corresponds with the fold mentioned in the 

 preceding stage as forming the beginning of the submeckelian fold 

 (Fig. 6, S.M.F.). 



This fold subdivides the antero-lateral wall into two surfaces, an 

 external, dorso-lateral and a mesial ventro-lateral surface. The former 

 forms an elongated triangular area limited dorsally by the tubo- 

 tympanal crest and posteriorly by the lateral ridge. The latter forms 

 a smaller triangle bounded internally by the alveolo-lingual fold and 

 posteriorly by the lateral ridge. 



The anterior prolongation of the tubo-tympanal ridge is more pro- 

 nounced than in the preceding stage. The difference is due to a con- 

 tinuance of the process, already mentioned, of anterior elongation of the 

 oral cavity. 



A pig of 12 mm. (No. 518, Harvard series. Figs. 9-11) shows the 

 pharynx only slightly larger than in the stage just described. The 

 continued anterior elongation of the oral cavity has given the tubo- 

 tympanal crest a decided antero-posterior course. The pouch retains 

 its connection with the ectoderm only at its dorsal apex. The lateral 

 ridge forms only a low prominence extending from the dorsal apex to 

 the ventral diverticulum. 



The dorsal apex appears 1)roader and more depressed than in the 

 preceding stage. This condition, I think, results from the lateral flex- 

 ure of the apex in consequence of the general growth in width of the 

 head. 



At this stage the pouch has the essential features of the primary 

 tympanic pouch of Kastschenko. This investigator considered the pouch 

 as merely a widened diverticulum of the lateral wall of the pharynx 

 and regarded the lateral ridge as alone representing the first phar}Tigeal 



