The Pharyngeal Pouches in the Mammalia 207 



angle it projects dorsalwards as a narrow prominence, the dorsal apex 

 (recessus anterior). From this apex three prominent ridges diverge, 

 i. e., antero-lateral, lateral and postero-lateral. The first extends 

 diagonally inwards and slightly forwards. It forms the sulcus tubo- 

 tympanicus of Moldenhauer. The lateral ridge is that by which attach- 

 ment to the ectoderm is effected. The area of attachment includes 

 nearly its entire extent. Yentrally this ridge is continued into the 

 ventral diverticulum. The postero-lateral ridge extends obliquely in- 

 wards and backwards from the dorsal apex to the dorsum of the second 

 pouch. 



The ventral diverticula of the first pair of pouches are at first more 

 prominent than those of the succeeding, but they are soon outstripped 

 by the latter. Typically they form a pair of low, but sharp folds, which 

 at first are continuous across the median line of the pharynx. 



Period II. The most important changes leading to the transforma- 

 tion of the first pouch into the primary tympanic pouch are the fol- 

 lowing : 



The gradual separation of the pouch from the ectoderm. This 

 process begins on the ventral side and progresses dorsalwards until 

 complete separation has been effected. In consequence of this separa- 

 tion the lateral ridge becomes greatly reduced and partly absorbed int"S 

 the neighboring walls of the pouch. 



The tubo-tympanal border becomes prolonged in the anterior direc- 

 tion. This change is produced as a result of the elongation in the same 

 direction of the adjacent part of the oral cavity. 



The ventral diverticula first become interrupted in the mid-line, and 

 later gradually disappear as a result of absorption into the fioor of the 

 pharynx. 



The basal or mesial portion of the pouch is displaced ventralwards in 

 consequence of a corresponding displacement in the adjacent part of tlte 

 pharynx itself. This portion of the pouch thus assumes an almost 

 horizontal position. At first the peripheral part, owing to its continued 

 attachment to the ectoderm, retains its ascending course, joining the 

 mesial portion at a sharp angle. Later, after complete separation from 

 the ectoderm, the peripheral portion also sinks, and thereby assumes a 

 plane more nearly like that of the mesial portion. 



The submeckelian fold is formed by the union of the dorsal remnant 

 of the lateral ridge with the diagonal fold separating the basal and 

 peripheral portions of the pouch. At first the fold is continuous an- 



