The Pharyngeal Pouches in the Mammalia 193 



The first pharyngeal pouch has a greater lateral extent than any of 

 the succeeding. As the figures show, the lateral extension of the pouches 

 decreases regularly from before backwards. The first pouch blends with 

 the lateral portion of the pharynx by a broad base, so that it is impos- 

 sible to draw any definite line between the two. The limits assigned by 

 Hammar, whose usage in this matter I adopt, will be given presently. 

 Laterally the outer extremity of the pouch is produced upwards as a 

 blunt prominence — the dorsal diverticulum of Hammar — which projects 

 considerably above the roof of the pharynx. The dorsal diverticulum 

 terminates in a narrow apex — ^the dorsal apex (d. a. 1) (Eecessus tym- 

 pani anterior, Hammar). 



Hammar includes in the first pouch the following parts: (1) The 

 sulcus tubo-tympanicum. This is Moldenhauer's term for the promi- 

 nent ridge (Fig. 2, S. T. T.) representing the antero-lateral border of 

 the pouch. It begins externally at the dorsal apex and extends down- 

 wards, inwards and forwards, terminating close to the base of the 

 hypophysis. (3) The latero-ventral ridge and its continuation, the 

 ventral diverticulum. This is a narrow ridge which in its dorso-lateral 

 portion is joined to the ectoderm. The connection includes the dorsal 

 two-thirds of its lateral extent. The lower third forms a free edge, and 

 this, at its lower outer angle, turns sharply inwards and backwards to 

 form the ventral diverticulum. (3) The sulcus tensoris tympani 

 (S. T. Ty.)- This is a term applied by Hammar to the border extending 

 from the dorsal apex backwards and inwards to join the next part along 

 the inner border of the hyoid arch. (4) The sulcus tympanicus pos- 

 terior (S. T. P.). This term, also given by Hammar, includes the longi- 

 tudinal ridge forming the inner boundary of the hyoid arch and con- 

 necting the sulcus tensoris tympani with the base of the second pouch. 

 (5) The impressio cochlearis. This Hammar defines as a conspicuous 

 depression on the dorsal wall of the pouch close to its origin from the 

 pharynx. The auditory sac lies immediately above this area. 



The second pharyngeal pouch is characterized, as already mentioned, 

 by the extensive contact of its lateral margin with the ectoderm. Only 

 at its extreme lower end does this border have a free margin. So far 

 as the present specimen is concerned, there is no communication between 

 the lumen of the pouch and the exterior. The closing membrane is 

 exceedingly thin, but examination shows no break in its continuity. 



The ventral diverticulum of this pouch forms a prominent quad- 

 rangular fold. The mesial half forms only a faint ridge, but the lateral 



