202 Henry Fox 



anterior extension and flexure of the mouth, and (3) the relative fixation 

 of the primary tympanic pouch b}' the differentiation of the surround- 

 ing cartilages. 



As a result of the flexures of the pharynx and mouth the common 

 structure now has the form of an arch (Fig. 23), the apex of the arch 

 heing that part lying between the primary tympanic pouches. From 

 . /. side of this part each pouch projects as a broad, flattened fold, 

 which towards its periphery turns strongly upwards so that the apex 

 again extends some distance above the roof of the pharynx. Together 

 the tubo-tyrapanal and tensor tympani borders form an arched curve, 

 the apex being formed by the dorsal apex (Fig. 23). The submeckelian 

 fold (S-M.F.) has much the same appearance as before. It is com- 

 pletely separated from the vestibular fold. It, however, no longer 

 projects below the ventral line of the pouch, but lies a slight distance 

 above it on the lateral surface. This change has been effected by a 

 process which only becomes noticeable in the region of the pouch at 

 this time. This is the downward growth and posterior extension of the 

 alveolo-lingual folds (A.L.F.). As these gTow down they carry with 

 them the adjacent ventro-lateral wall of the pouch, and thus the lat- 

 ter loses its original horizontality and assumes an inclined position. Its 

 surface thus comes to be more nearly continuous with the plane of the 

 dorso-lateral portion. Since the submeckelian fold forms the dividing 

 line between the two portions, it comes, in consequence of this change, 

 to occupy its present relatively higher level on the side of the pouch. 



An important, but at this stage inconspicuous, feature is a shallow 

 indentation on the posterior tympanal rim between it and the second 

 pouch (see Figs. 26-27, z.). The latter pouch is now so small that the 

 exact line of demarcation is not easily recognizable. A slight ridge 

 (Fig. 26, p-m.f.), however, which extends from the indentation to 

 the submeckelian fold, enables one to fix upon this point as being 

 between the two structures. The same ridge, showing the same rela- 

 tions, is present in the immediately preceding stage when the second 

 pouch was still clearly recognizable. This ridge later becomes the 

 prominent elevation limiting the manubrial fossa posteriorly. 



In the cat of 10.7 mm. (No. 474, Harvard series) the tympanic 

 pouch shows a slight advance. The indentation between the pouch and 

 the second pouch is slightly deeper and consequently the posterior 

 tympanal crest now forms a rounded lobe projecting dorsally. In all 

 other respects this stage is so similar to the preceding that further 

 description is unnecessary. 



