200 Henry Fox 



tinctly indicated by low swellings on the under side of both pouch and 

 pharynx. The dorsal half of the lateral ridge, however, is continued 

 into the submeckelian fold, and these are now slightly more prominent. 

 Together they now form a continuous crescent-shaped fold extending 

 from the dorsal apex to the base of the vestibular fold. It underlies, 

 for the greater part of its length, Meckel's cartilage. For this reason 

 I have called it the submeckelian fold. The shallow depression in the 

 lateral wall which it subtends I call the Meckelian fossa. 



The paired ridges which formerly limited the tubercalum impar 

 laterally have now become blended with the epithelium covering the 

 tongue anlage. The formation of the latter has been accompanied by 

 the progressive downgrowth of the surrounding alveolo-lingual crests, 

 particularly in their anterior portion. The deep space thus enclosed is 

 filled with the tissues of the organ. Posteriorly this space is now con- 

 nected by a deep groove with the space in front of the larynx. 



An early stage in the formation of the external auditory meatus 

 is shown by the conical indentation projecting under the pouch. Its 

 inner angle terminates a short distance below the latero-posterior sur- 

 face. The two structures are nowhere in contact, a moderately thick 

 layer of mesenchyme intervening between them. 



In the pig of 17 mm. (No. 51, Harvard series. Figs. 19-21) the 

 primary tympanic pouch is slightly more expanded and depressed. The 

 dorsal apex has become flattened out to a low rounded prominence and 

 has sunken to a lower level, so that it scarcely projects above the level 

 of the pharyngeal roof. The tubo-tympanal crest in consequence is 

 almost horizontal. Anteriorly it turns sharply inwards to form the 

 relatively short tubal portion, the remainder forming the tympanic part 

 (see Fig. 20). 



On the lateral wall the submeckelian fold forms a prominent, pro- 

 jecting ridge. It extends from the dorsal apex downwards and for- 

 wards to the latero-inferior edge, when it projects as a convex ventral 

 pocket. In front of this region it suddenly dies out, forming only a low 

 fold (Fig. 19, y.), continued to the base of the vestibular fold. The 

 interval outside of this part is occupied by Meckel's cartilage. The 

 latter ascends from the mandibular arch in the angle between the sub- 

 meckelian and the vestibular folds and thereby comes to lie in front 

 of and above the former. The presence of the cartilage in the angle 

 mentioned has probably some close connection with the separation of 

 the two folds. Its presence would inhibit continued lateral extension 



