204 Henry Fox 



On the expanded lateral wall of the tympanic ponch two prominent 

 outstanding folds are now present. The anterior is the snbmeckelian 

 fold; it shows the now deep Meckelian fossa on its dorsal side. The 

 posterior fold is less prominent; it corresponds to the ridge formerly 

 mentioned as forming the posterior limit of the manubrial fossa. The 

 latter now forms a depression of considerable depth. 



A rabbit of IGi/s days, 17.8 mm. (Ko. 576, Harvard series. Fig. 71) 

 shows the constriction of the tympanic pouch still further progressed. 

 The post-salpingeal fold (p. s. f.) is more convex. The dorsum of the 

 second pouch (ton. f.) is separated by a short interval from the base 

 of the tympanic pouch. The remaining features of the pouch are 

 essentially like those in the following stage. 



This stage is represented by a pig of 24 mm. (No. 64, Harvard series, 

 Figs. 29, 30). The constriction of the tympanic pouch has now reached 

 a stage where its connection with the pharynx embraces about two- 

 thirds of its former extent. The ventro-mesial margin is accordingly 

 of considerable length. The posterior half of the poiich projects strongly 

 backwards as a wide, cup-shaped fold. 



The submeckelian fold forms a wide, almost horizontal shelf (Tigs. 

 47-50, s. m. f.). Laterally it reaches considerably beyond the dorsal 

 apex, so that it is clearly visible from above (Fig. 30). On lateral 

 view it appears at a considerably higher level than before. This posi- 

 tion it has obtained partly as a result of its lateral extension and the 

 consequent flattening of its dorsal surface and partly from the ventral 

 downgrowth of that portion of the pouch lying immediately below it 

 (cf. Figs. 48-50, with Fig. 43). 



The manubrial fossa forms a shallow impression between the sub- 

 meckelian (s. m. f.) and post-manubrial folds (p. m. f.). The latter is 

 much less prominent in the pig than in the equivalent stage of the cat. 



The pig of 25 mm. (Series M^, my collection), while slightly more 

 advanced than the preceding, is essentially similar so far as the tym- 

 panic pouch is concerned. Figs. 47-51 give views of several transverse 

 sections of the structure. 



The next step in advance is shown by a cat of 23.1 mm. (No. 466, 

 Harvard collection. Figs. 66-67). In this case the tympanic pouch 

 and Eustachian tube are first clearly differentiated from each other. 

 The former is a wide, cup-shaped expanse, concave dorsally. As a 

 whole, it has a decided ascending direction. The postero-lateral border 

 (P-L.B.) forms a highly elevated ridge. Immediately back of the 



