The Pharyngeal Pouches in the Mammalia 199 



pouch. As Hanimar shows and my observations confirm, Kastschenko's 

 conception of the pouch was entirely too limited and was doubtless due 

 to his not examining earlier stages in which it is more typically 

 developed. 



The primary tympanic pouch at this stage is a dorso-ventrally flat- 

 tened triangular fold which arises by a broad base from the pharynx 

 and terminates peripherally in the dorsal apex. The pouch as a 

 whole lies almost horizontally, but towards the lateral edge it turns 

 sharply upwards. Its walls are medio-dorsal and lateral. The former 

 is limited laterally by the tubo-tympanal crest, dorsal apex and pos- 

 terior tympanal borders. All below these limits is embraced in the 

 lateral wall. This is divided by the lateral ridge into two surfaces, 

 antero-lateral and latero-posterior. The antero-lateral surface is fur- 

 ther subdivided into two areas — dorso-lateral and ventro-lateral — by the 

 submeckelian fold. 



The ventral diverticula now form a pair of low swellings. Mesially 

 they are interrupted by a shallow longitudinal groove connecting the 

 tongue concavity with the deep hollow in front of the larynx. 



In a pig of 13.5 mm. (Series M^ of my collection) the condition of 

 the pouch is intermediate between that last described and the next. 

 The only feature that calls for remark is the presence at the dorsal 

 apex of a short narrow process by which the pouch retains its last con- 

 nection with the ectoderm (Fig. 37). 



In the pig of 14 mm. (No. 65, Harvard series. Figs. 14-16) the 

 primary tympanic pouch has separated entirely from the ectoderm and 

 now lies some distance below it — a condition due to the greater lateral 

 growth of the head compared with that of the pouch. 



The pharynx at this time begins to show modifications due to its own 

 differential growth. The increase in width of the anterior half is 

 considerably greater than in the posterior portion. Thus, while the 

 distance between the apices of the first pair of pouches has increased 

 appreciably since the last stage, that between the same parts in the 

 second pair remains approximately the same. In consequence of this 

 the pouch now shows a more pronounced lateral projection. The tensor- 

 tympani crest turns sharply inwards and joins the posterior tympanal 

 border at an obtuse angle. The latter border also shows a tendency to 

 assume a more transverse trend. The second pouch appears as a rounded 

 prominence at the postero-internal angle of the tympanic pouch. 



The ventral half of the lateral ridge and its continuation, the ventral 

 diverticulum, have disappeared. Their former position is only indis- 



