The Pharyngeal Pouches in the ]\Iammalia 203 



The cat of 15 mm. (Xo. 436, Harvard series, Fig. 63, 64) presents 

 the next stage in the modification of the tympanic pouch. The incision, 

 which in the preceding stages had just begun to form Ijetween the 

 tympanic poucli and the dorsum of the second pouch, is now much 

 deeper. The postero-Iateral lobe of the pouch in c<.«-Lsequence pro- 

 trudes more strongly in the dorso-posterior direction '^ 'iks a result of 

 the incision a new ventro-mesial border (Y.M.E.) has begun to form 

 between the base of the pouch and the pharynx. Posteriorly this border 

 connects by a rounded angle with the posterior tympanal border 

 (Fig. 64, s. t. p.). 



The connection of the pouch with the pharynx is both relatively and 

 actually of lesser extent than in the preceding stages. This condition 

 represents the commencement of the gradual constriction of the con- 

 necting part as a result of the anterior extension of the incision. 



Owing to the increased depth of the intervening incision the tympanic 

 pouch is now completely separated from the second pouch. This condi- 

 tion is apparently produced in the following manner : It will be recalled 

 that the pouch has now become relatively fixed in position by being 

 included between Meckel's cartilage with its manubrial process and the 

 auditory capsule. The neighboring lateral walls of the pharynx, on the 

 other hand, are continuously being displaced to a lower level by the 

 downgrowth of the alveolo-lingual margins. Among the parts thus 

 carried down is the concavo-convex fold representing the dorsal remnant 

 of the second pouch (Ton.F.). In consequence of this displacement of the 

 second pouch and the relative fixity of the tympanic pouch, the incision 

 (z) between the two spreads dorsally over the second pouch and reaches 

 the longitudinal ridge (P-S.F.) lying immediately internal to the base of 

 the pouch (cf. Figs. 26, 70 and 64). Thus the base of the tympanic 

 pouch is placed in connection with this ridge, which is the extel-nal 

 expression of the groove extending backward from the Eustachian open- 

 ing between the levator cushion and the salpingo-pharyngeal fold. For 

 convenience in description I shall speak of it as the post-salpingeal 

 groove. 



As a result of the process just described the dorsum of the second 

 pouch comes to lie on the lateral surface of the pharynx below the 

 posterior margin of the tympanic pouch. With the subsequent down- 

 growth of the alveolo-lingual folds it is carried farther ventralwards, 

 and, as will be described later, is finally transformed into the tonsillar 

 recess. 



