The Pharyngeal Pouches in the Mammalia 197 



gives a ventral view in the latter. The first pharyngeal pouch is wider 

 in the antero-posterior direction than before — a change connected withr 

 the anterior growth and elongation of the oral cavity and the conse- 

 quent prolongation in the same direction of the attached tubo-tym- 

 panal rim. The ventral diverticula are slightly less prominent. To- 

 gether they form a low V-shaped elevation on the floor of the pharynx. 

 Just external to the median apex formed by the convergence of the 

 arms of the V each is joined by one of the pair of folds forming the 

 outer line of the tuberculum impar (Tub.). Close to the lateral margin 

 each arm is crossed by the broad alveolo-lingual fold (AL.F.). A 

 slight distance in front the latter meets the vestibular fold (V.F.). 

 Immediately back of the point of convergence a lateral fold (SM.F.). 

 is given off, which extends obliquely outwards and backwards over the 

 lateral ridge of the latter. The formation of this fold marks the initial 

 step in the development of the later important submeckelian fold. 



The tuberculum impar arises as a result of the bipartition of the 

 median oral ridge. The crest of the latter widens and its middle part 

 then becomes depressed to form a shallow concavity — the ventral coun- 

 terpart of the tuberculum. 



In the pig of 10 mm. (No. 401, Harvard series. Figs. 4-6) the pouch 

 is joined to the ectoderm by only the dorsal third of its lateral ridge. 

 The remainder of this border is now free and forms a low fold sepa- 

 rating the antero-lateral and postero-lateral surfaces of the pouch. Ven- 

 trally it is continuous with the ventral diverticulum. Where the transi- 

 tion takes place the alveolo-lingual swelling cuts across it at right 

 angles, forming here the line of demarcation between the pouch and 

 the pharjTix. 



The ventral diverticula present no new points of interest. The 

 swellings which marked the lateral boundaries of the tuberculum impar 

 are now relatively inconspicuous, having been absorbed along with the 

 adjacent parts of the pharyngeal floor in the broad depression (repre- 

 senting the anlage of the tongue) lying between the alveola-lingual 

 ridges. 



Dorsally the pouch projects relatively higher than hitherto and ter- 

 minates in a more acute apex. This condition is not due to the growth 

 dorsalwards of the pouch, but is a result of a ventral displacement of 

 the phar}Tix. As a comparison of the figures shows, the formation of 

 the neck of the animal is attended with a ventral (caudal) flexure of 

 the posterior half of the pharynx. The flexure also affects to a minor 



