The Pharyngeal Pouches in the Mammalia 215 



The dorsal apex has been absorbed into the neighboring surface of 

 the dorsal prominence. 



The ventral diverticulum forms only an inconspicuous fold in the 

 same situation as hitherto. 



(b") The Formation of the Tonsillar Fold. 



The cat of 15 mm. (ISTo. 436, Harvard series, Figs. 63-64) gives us 

 the initial step in the transformation of the remnant of the second 

 pouch into the tonsillar fold. 



In this stage the dorso-lateral fold, representing the second pouch, 

 no longer forms a continuum with the adjacent border of the tympanic 

 pouch, but is separated from the latter by an indentation which extends 

 quite across its dorsal side to the longitudinal ridge (P.S.F.), forming 

 its mesial boundary. 



The second pouch thus forms an arched lateral fold. The lateral 

 margin (Ton.F.) of the fold lies on a level with the ventro-lateral line of 

 the pharynx. The ventral side is concave; the dorsal correspondingly 

 convex. Anteriorly the fold is continued immediately under the ventro- 

 internal angle of the tympanic pouch and extends to the base of the 

 vestibular fold. Internally the fold is limited on the ventral side by 

 the alveolo-lingual fold, on the dorsal by the adjacent surface of the 

 pharynx. The structure thus defined is the tonsillar fold (tonsillar 

 sinus). The concavity on its ventral side corresponds to the tonsillar 

 prominence (tonsillenhocker). 



There is no trace at this stage of the filiform process. 



The rabbit of I6I/2 days (No. 576, Harvard series. Fig. 71) shows an 

 essentiall}' similar condition. The fold (ton. f.) is more strongly 

 arched and its lateral edge lies some distance above the lower edge of 

 the alveolo-lingual groove. The fold is widest immediately imder the 

 post-salpingeal ridge (p. s. f.). Its anterior continuation forms a low 

 ridge, which probably represents an extension of the fold over the 

 adjacent surface of the pharynx. 



In the 24 mm. pig (No. 64, Harvard series. Figs. 29-30) the tonsillar 

 fold is removed by a considerable interval from the base of the tympanic 

 pouch (Fig. 30). Between them the surface of the pharynx is de- 

 pressed, forming the palatal constriction. The present position of the 

 fold is due to the formation of this constriction and to the continued 

 downgrowth of the alveolo-lingual margin (A-L.F.) with which it is 

 closely associated. 



