206 Henry Fox 



the tympanic ponch is widel}^ expanded, particularly in its posterior 

 part. It still retains its cup-like form, the concave surface fitting 

 closely against the ventro-lateral wall of the auditory capsule. The 

 submeckelian fold (S-M.F.) is relatively not so prominent as earlier. 

 The dorsal apex or anterior recess (D.A.I) projects strongly outwards. 

 The manuljrial fossa (Mn.F.) forms a deep hollow on the more dorsal 

 portion of the lateral surface. It is largely surrounded by the now 

 high and conspicuous post-manubrial fold (P.M.F.). Below the fossa 

 is the surface already mentioned as being in close relation with the 

 external auditory tube. The remaining posterior extension of the pouch 

 is simply applied to the neighboring part of the auditory capsule. 



The latest stage studied is shown by a rabbit of twenty-one days. 

 Fig. 73 shows the more important points. The manubrial fossa 

 (]\In.F.) is still rather deep in its dorsal half, but vertrally becomes 

 very shallow and there tapers out to a point. The area entering into 

 the constitution of the tympanic membrane is more extensive than in 

 the preceding stage. It now includes a considerable part of the surface 

 in which tbe manubrial fossa is located. 



The ]Meckelian fossa (Mk.F.) is almost ol^literated ; it persists as a 

 very shallow impression on the antero-dorsal margin close to the union 

 of the pouch with the Eustachian tube. 



Review and Comparisons. 



The foregoing results render it highly probable that the develop- 

 mental history of the first pharyngeal pouch is essentially the same in 

 the three species of mammals studied. 



This history I have subdivided into three periods, as follows : 



(1) The period of formation of the typical pouch. 



(2) The period of transformation of the pouch into the primary 

 tympanic jjouch. 



(3) The period of differentiation of the tympanic pouch and Eustach- 

 ian tube and their subsequent modifications. 



Period I. The formation of the pharnygeal pouches takes place in 

 the usual manner — beginning with the most anterior and ending with 

 the most posterior. 



When typically developed the first pharyngeal pouch has the form 

 of an approximately transverse vertical fold. At its dorsal lateral 



