DEVELOPMENT OF THE SENSE ORGANS 39I 



counts of the transformation of these hillocks into the adult auricle agree in the 

 main. 



Caudal to the hyoid anlages a fold of the hyoid integument is formed, the 

 auricular fold or hyoid helix. A similar fold forms later, dorsal to the first branch- 

 ial groove, and unites with the auricular fold to form with it the free margin of the 

 auricle. The point of fusion of these two folds marks the position of the satyr 

 tubercle, according to Schwalbe. Darwin's tubercle appears at about the middle of 

 the margin of the free auricular fold, and corresponds to the apex of the auricle in 

 lower mammals. The tragus is derived from mandibular hillock 1 ; the helix from 

 mandibular hillocks 2 and 3; the antihelix from hyoid hillocks 4 and 5; the anti- 

 tragus from hyoid hillock 6. The lobule represents the lower end of the auricular 

 fold. 



The external auditory meatus is formed as an ingrowth of the first branchial 

 groove. In embryos of 12 to 15 mm. the wall of this groove is in contact dorsally 

 with the entoderm of the first pharyngeal pouch. Later, however, this contact 

 is lost, and during the latter part of the second month, according to Hammar, 

 an ingrowth takes place from the ventral portion of the groove, to form a funnel- 

 shaped canal. 



The lumen of this tube is temporarily closed during the fourth and fifth months, but 

 later re-opens. During the third month a cellular plate at the extremity of the primary 

 auditory meatus grows in and reaches the outer end of the tympanic cavity. During the 

 seventh month a space is formed by the splitting of this plate, and the secondary inner por- 

 tion of the external meatus is thus developed. 



The tympanic membrane is formed by a thinning out of the mesodermal tissue 

 in the region where the wall of the external auditory meatus abuts upon the wall 

 of the tjonpanic cavity. Hence it is covered externally by ectodermal, and inter- 

 nally by entodermal epithelium. 



