218 Henry Fox 



sillar projection. The closely adpressed dorsal wall is correspondingly 

 raised into a dome-shaped swelling, the dorsal prominence. 



After its separation from the ectoderm the original lateral margin of 

 the pouch recedes towards the median line. At first it forms a slight 

 projection at the postero-internal angle of the pouch, but later this is 

 absorbed and then forms a continuum with the dorso-lateral fold. 



The ventral diverticulum early diminishes in size and later is absorbed 

 by the downgrowth of the alveolo-lingual fold. 



At the termination of the first period the remains of the second pouch 

 form a laterally ascending arched fold, which lies at the postero-internal 

 angle of the tympanic pouch. On its ventral side it is concave and on 

 the dorsal correspondingly convex. Its inner boundary is formed by 

 the alveolo-lingual ridge. 



The second or progressive stage is marked (1) by the separation of 

 the second pouch from the tympanic pouch, (3) its ventral displace- 

 ment to its definitive position, and (3) its progressive modification to 

 form the tonsillar fold. 



The separation from the tympanic pouch takes place by the extension 

 of the indentation between the two structures over the dorso-lateral 

 surface. The second pouch thus comes to lie at a slightly lower level 

 than the base of the tympanic pouch. 



The ventral displacement takes place in connection with the con- 

 tinued downgrowth of the alveolo-lingual ridge and the accompanying 

 formation of the palatal constriction. The latter forms between the 

 base of the tympanic pouch and the dorsum of the tonsillar fold and as 

 it enlarges the fold is pushed farther ventralwards, where it attains its 

 final definitive position. 



In its principal features the tonsillar fold in the later embryonic 

 stages is similar in the species studied. It then forms a prominent 

 arched fold on the lateral surface of the oral portion of the pharynx 

 parallel to the alveolo-lingual sinus. Its ventro-lateral surface is con- 

 cave, its dorsal convex. 



The later modifications are concerned with the assumption of its 

 definitive form. Owing to lack of materials, these modifications were 

 not traced. The rabbit of 21 days shows that they may be consid- 

 erable. I shall consider them further in the comparative part. 



Contributions to the developmental history of the second pouch have 

 been made by a number of investigators, chief among whom are Born, 

 His, Rabl, Piersol, Kastschenko and Hammar. The results of these 



