The Pharyngeal Pouches in the Mammalia 201 



of the connecting portion, while it would not interfere with such 

 growth in the remainder of the fold. The latter would then continue 

 to expand laterally and would thus give rise to the prominent projec- 

 tion which it forms at this stage. 



The form of the pharynx is essentially the same in a pig of 18 mm. 

 (Series M% my collection) and a rabbit of 14 days (No. 157, Harvard 

 series. Fig. 70). It differs but slightly from that last described. The 

 greater part of the tympanic pouch lies in an almost horizontal plane, 

 only its extreme lateral portion being slightly upturned (Figs. 41-46). 

 The dorsal apex forms only a low eminence, the meeting point of tubo- 

 tympanal border, submeckelian fold and tensor tympani border. 



The most important feature of this stage consists in the definite 

 segregation of the neighboring skeletal structures, particulary Meckel's 

 cartilage and the auditory capsule. Their formation is so intimately 

 associated with certain later modifications of the pouch that a short 

 description of their essential characteristics is necessary. Meckel's 

 cartilage (Mck.) is a stout rod, which, as already mentioned, rises from 

 the mandibular arch in the angle between the submeckelian and vesti- 

 bular folds and then turns obliquely backwards above the former fold 

 (Figs. 41-43). Close to the posterior margin of the fold it sends down 

 the stout manubrium which curves around the back of the fold and 

 terminates in a slight depression — the manubrial fossa — immediately 

 beneath (Figs. 43-44). The submeckelian fold is thus wedged in the 

 angle between Meckel's cartilage and the manubrium and is thus rela- 

 tively fixed in position (Fig. 43). This relative fixity of the fold is 

 an important factor in the final transformation of the pouch into the 

 definitive tympanic pouch and Eustachian tube. 



The auditory capsule occupies the depression (Impressio cochlearis) 

 between the dorso-internal surface of the pouch and the roof of the 

 pharynx. 



(a") The Differentiation of the Tympanic Pouch and Eustachian 

 Tube. 



In a pig of 20 mm. (No. 542, Harvard series, Figs. 23-27) we 

 observe the beginning of the changes leading to the final transformation 

 of the primary tympanic pouch into the definitive pouch and Eustachian 

 tube. The transformation appears to be closely connected with a con- 

 tinuance of the processes already indicated. Of these we may recall 

 (1) the ventral (caudal) flexure and elongation of the posterior half 

 of the pharynx in connection with the formation of the neck, (2) the 



