1901.] NATURAL SCIENCES OF PHILADELPHIA. 223 



THE DEVELOPMENT OF THE TYMPANO-EITSTACHIAN PASSAGE AND 



ASSOCIATED STRUCTURES IN THE COMMON TOAD 



(BUFO LENTIGINOSUS). 



BY HENRY FOX. 



A perusal of the literature relating to the subject reveals the 

 Existence of considerable diversity of opinion among investigators 

 as to the exact morphological significance of the tympano-Eusta- 

 chian passage of the higher vertebrates. So far as its adult struc- 

 ture and relations are concerned, the passage would seem to be the 

 homologue of the spiracle or hyomandibular cleft of the elasmo- 

 brauch fishes. Both structures occupy the same relative position 

 between the mandibular and hyoid arches, and, moreover, above the 

 dorsal margin of each the facial nerve divides into its two main 

 branches, one of which, the ramus palatiuus, courses in front of 

 the cleft (or tube, as in the higher forms), while the other, the 

 ramus hyomandibularis, extends ventrally along its posterior wall. 

 Embryologists, however, in studying the development of the 

 tympano-Eustachian passage in various species of the higher verte- 

 brates, have found that its homology with the hyomandibular cleft 

 is not so clearly expressed as the mature structure of the organ 

 would lead one to infer, so that certain morphologists, basing their 

 conclusions on the facts revealed by embryology, hold that the 

 tympano-Eustachian passage is a structure entirely, or in large 

 part, independent of the hyomandibular cleft. 



In order to determine, if possible, the exact relation of the 

 tympano-Eustachian passage to the hyomandibular cleft, I under- 

 took to follow out its entire embryonic history in the common toad 

 of ihe eastern United States, Bufo lentighiosus.^ Contributions to 

 the knowledge of the development of the structures under con- 

 sideration had been made in the case of the Anura by Goette,^ 



^ The investigations have been made in the Zoological Laboratory of the 

 University of Pennsylvania. 

 ^ Entwkklungsgcschichte der Unke, Bomhinator igneus, Leipzig, 1875. 



