130 TROCEEDINGS OF THE AMERICAN ACADEMY 



means of the first cleft ; in other words, the posterior (lower, Mold.) 

 region of this sulcus is identical with the inner end of the fissure, which 

 in turn is continuous with the pharyngeal cavity. The question, then, 

 simply becomes. Which portion of this sulcus is converted into the 

 Eustachian tube and drum, — the anterior or posterior ? When it is 

 observed that the greatest lateral extent of the sulcus is encountered in 

 the cross sections (Moldcnhauer's Figs. 23, 25) which lie in the plane 

 of the branchial furrow, I think there need be no hesitancy in admit- 

 ting ihiit it is the posterior part of this sulcus which persists, and is im- 

 mediately concerned in the formation of the drum and tuba ; and if so, 

 there can be no valid objection to considering these spaces as resulting 

 from the metamorphosis of the inner edge of the first visceral cleft. 



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