The Pharyngeal Pouches in the Mammalia 213 



figiires (12 of last stage and 17 of this) show, this growth has not 

 been accompanied by a corresponding increase in the pouch, which at 

 this stage remains of the same width as in the preceding stage. Conse- 

 quently, as the second pharyngeal groove is displaced more and more 

 lateralwards, the attached ventro-lateral angle becomes drawn out into 

 the process here shown. Owing to its form, I designate the latter the 

 filiform process (fl. p.). 



The lateral margin of the pouch is considerably less prominent than 

 hitherto. This change appears to be produced by an actual regression 

 of the margin. This is indicated by the fact that the distance between 

 the lateral margins of the two opposite pouches is slightly less than in 

 the preceding stage. The regression is probably attributable to the 

 tension exerted upon this margin by the continued lateral extension of 

 the adjoining antero-lateral margin with which it now joins at a wid'S 

 angle. 



As just mentioned, the antero-lateral margin (ton. f.) has continued 

 to extend in the lateral direction. It thus has a decided antero-lateral 

 course. Por this reason it is inappropriate to call it by the term 

 hitherto used, and accordingly I shall hereafter speak of it as the dorso- 

 lateral margin. 



The dorsal apex (D.A.2) of the pouch now forms only a slight pro- 

 tuberance at the posterior extremity of this margin (Fig. 15). 



In consequence of the extension laterally of the dorso-lateral margin 

 the underlying ventro-lateral surface has acquired the form of a deep 

 concavity (Fig. 17, c. v.). The overlying dorsal wall is correspondingly 

 raised as a broad dome-shaped prominence (Fig. 15, D.Pr.). 



The ventral diverticulum (v. d. 2) has almost ceased to exist as a 

 distinct feature. Only in its more peripheral part does it project to a 

 fair degree below the ventro-lateral line of the pharynx. Its middle 

 part has largely disappeared owing to the downgrowth of the alveolo- 

 lingual ridge (Fig. 17, al. f.) and the union of the latter with the 

 sinus piriformis (Fig. 17, s. pi.). The place of the original divertic- 

 ulum is indicated by a widening of the continuous ventro-lateral fold 

 thus formed. 



The more internal part of the diverticulum persists as a slight ridge 

 on the inner side of the ventro-lateral fold (Fig. 17). 



In the 17 mm. pig (Figs. 19-22) the second pouch has entirely 

 severed its connection with the ectoderm, leaving the filiform process 

 terminating l^lindly in the mesenchyme. 



