136 



DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



which consists of parts corresponding to the elements of the auricle. The most 

 conspicuous of these are the scapha-helix, tragus, and antitragus. The angular 

 fossa has the form of an elongated depression. Its anterior margin is entirely of 

 mandibular origin and its posterior margin of hyoid origin. The greater part of 

 this posterior margin is taken up with the scapha-helix, or the so-called free ear-fold, 

 at the lower end of which is the modified remnant of hillock 6, which persists as the 

 antitragus. Hillock 1 has become directly converted into the tragus, whereas the 

 crus is slower in making its appearance; not until the embryo has reached a length 

 of 18 or 20 mm. does this structure become evident. It arises from the mandibular 

 tissue in the region formerly occupied by hillocks 2 and 3 and forms an oblique 

 ridge which, enlarging, encroaches upon the angular fossa and converts it into a 

 narrow cleft. 



The transformation from the more primitive type of auricle, as just described, 

 into the adult ear may be easily followed in figure 8. This figure is intended as a 

 diagrammatic analysis of the changes illustrated by the photographs on plates 4, 



/ M- 





23mm. 



/ 



4) mm 



a 



I m 





85" 



135 ' 



Fio. 8.- 



-Drawings showing the development of the auricle and its primitive form to the adult type. Those parts derived 

 from the mandibular bar are indicated in lighter tone and are relatively larger in the younger stages; the parts 

 derived from the hyoid bar are stippled; the broken line represents the approximate junction of the anthelix 

 and scapha-helix. 



5, and 6. The parts of the auricle derived from the mandibular bar are shown in 

 a fighter tone, while the parts derived from the hyoid bar are stippled. It is inter- 

 esting to note that the mandibular derivatives are relatively very large in the earlier 

 stages, and this is also true of the derivatives of the lower end of the hyoid bar; 

 in other words, those parts of the auricle concerned with the closure mechanism 

 and the attachment of the auricle to the head are more precocious than the scapha- 

 helix and anthelix. The latter two structures merge directly into each other. 

 Their approximate point of junction, however, is indicated by a dotted line. 



In the younger stages for example, 85 mm. the soft tissues of the auricle 

 give the appearance of fulness and tend to be thrown into folds. These should not 

 be confused with the longitudinal folds seen in the adult scapha of some of the long- 

 eared animals. As the cartilage expands, the subcutaneous tissue becomes rela- 

 tively more scant, particularly in the region of the anthelix and scapha-helix. The 

 histological appearance of the crus helicis is quite different from the hyoid auricle 

 (anthelix and scapha-helix). This difference consists chiefly in the presence of a 

 great number of hair follicles and a considerable amount of subcutaneous fat. 

 These are almost absent in the scapha-helix. The form of the concha, particularly 



