The Pharyngeal Pouches in the Mammalia 211 



posterior and dorsal. The antero-Iateral border (Ton.F.) extends from 

 the dorsal apex diagonally forwards and inwards to the postero-internal 

 angle of the first pharyngeal pouch. It forms the dividing line between 

 the dorsal and antero-Iateral surfaces. The postero-internal border is 

 approximately crescentiform. Laterally, owing to the posterior flexure 

 of the pouch, its course is almost longitudinal, but basally it bends first 

 inwards and then posteriorly and connects with the lateral ridge ex- 

 tending to the third pouch. It separates the dorsal and postero-internal 

 surfaces. The lateral margin forms the part connected with the ecto- 

 derm. It separates the antero-Iateral and postero-internal surfaces. 

 Ventrally it is continued into the ventral margin, which forms the free 

 edge of the ventral diverticulum. 



In a cat of 9.7 mm. (ISTo. 446, Harvard series, Fig. 61) the second 

 pouch, beyond a slight increase in size, shows but few new features. 

 For a short distance below the dorsal apex it has separated from the 

 ectoderm — the initial step in the process which in this case begins at 

 the dorsal end and progresses- towards the ventral. The separation is 

 accompanied by the ingrowth of mesenchyme into the intervening space. 



The latero-ventral angle of the ventral diverticulum is produced into 

 a slight projection. As a result the inner border of the diverticulum 

 ascends more diagonally to the floor of the pharynx. 



In the pig of 10 mm. (No. 401, Harvard series) a departure from 

 the preceding stage is shown by the slightly lower level of the second 

 pouch. In the preceding examples the dorsal line of the pouch lay a 

 short distance above the same line of the pharynx, while in the present 

 stage it lies below it. This condition is probably produced by the 

 changes taking place in the neighboring parts. The hyoid region 

 increases in thickness more rapidly in its dorsal portion than in its 

 relatively passive ventral part. The dorsal portion thus projects out- 

 wards over the lower and consequently the second ectodermal groove 

 assumes a more inclined direction than before. With the latero-ventral 

 rotation which the dorsal half of the groove undergoes it naturally 

 results that the attached dorsal portion of the internal pouch accom- 

 panies it, at least in part, in the same direction, and thus assumes a 

 more lateral, as well as lower, position. 



In the 12 mm. pig (N"o. 518, Harvard series. Figs. 9-12) the second 

 pouch comes to a standstill so far as any lateral growth is concerned. 

 Thus the distance between the lateral margins of the two opposite 

 pouches remains the same as in the preceding stage. The first pharyn- 

 geal pouch, on the contrary, continues to extend rapidly in that direc- 



