DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



125 



In embryos of 8 to 11 mm. the component parts of the mandible have begun 

 to express themselves, and with their increase in size there is a corresponding 

 spreading apart of the auricular areas of the two sides, as shown in figure 10. This 

 drawing was made from a reconstruction model, which, because of the development 

 of the auricular hillocks, shows very clearly the surface area that enters into the 

 formation of the external ear. In the 6-mm. embryo we were dealing with a series 

 of four simple branchial bars; here these bars have partially lost their identity. 

 The first two have undergone marked development, whereas the third and fourth 

 have become much less conspicuous. The first or mandibular portion is broken 

 up into (1) the part that will form the lower jaw and (2) a more lateral part, whose 

 surface forms the three mandibular auricular hillocks, two of which can be seen 

 in the figure. These hillocks and those of the hyoid bar have caused deflections in 

 the first gill-cleft, whose ventral termination will eventually be represented by the 



meat aud c\ i. 



Flos. 3 and 4. Figure 3 shows (a) ventral and (b) lateral views of an agnathous specimen illustrated in Forster's Atlas of 

 Malformations, 1865 (plate 13, figs. 19 and 20). Figure 4 is copied from a case of cyclopia and agnathia from 

 the Pathological Institute at Heidelberg, described by Schwalbe (1909, p. 615). In both of these cases there 

 is a complete arrest in the development of the greater part of the mandibular arch, with the result that the 

 auricles retain their original median position. 



intertragal incisure. The surface of the hyoid bar is entirely taken up with its 

 three auricular hillocks, all of which show in the figure. The small third branchial 

 bar can be seen partly exposed, but the fourth is entirely covered in. 



Between the stages of 10 and 14 mm. there is rapid progress in the formation 

 of the face, as can be seen by comparing figures 10 and 11. Figure 11 is drawn from 

 a model to show the details of the face region and the topography of the auricular 

 area, the latter shown in blue. The mouth at this time is fairly well outlined, and 

 one can recognize the region between it and the auricular area which is to form the 

 cheek and jaw. As this region enlarges it will result in the further lateral and 

 dorsal displacement of the auricular area. In the preceding stages the latter still 

 extended downward on the ventral surface of the head, whereas now it is entirely 

 on the lateral surface, and the whole area can be seen in a profile view of the embryo. 

 At this stage the six auricular hillocks show their maximum prominence. The 

 three mandibular hillocks, which at first covered a large part of the mandibular bar, 

 now cover only its caudal margin. The three hyoid hillocks still represent the whole 

 surface of the hyoid bar excepting that part which has been molded into the first 

 cleft. It can be seen in figure 11 that this cleft is much wider than in the younger 



