124 DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



TOPOGRAPHY. 



In very young embryos (up to 12 mm. long) the branchial area involved in the 

 formation of the external ear constitutes a considerable portion of the ventrolateral 

 surface of the head. Growth in this area is precocious and in advance of the sur- 

 rounding structures. As the elements of the face and cranium later undergo 

 differentiation, the auricular area becomes relatively smaller, and at the same time 

 it appears to migrate dorsolateral^ from near the median line, until it finally occu- 

 pies its adult site on the lateral surface of the head. The transition in relative size 

 and position of these structures can be traced through figures 9 to 12 (plate 1), in 

 which the auricular area at different stages is shown in blue. 



If we start with the primitive branchial arrangement existing in a 6-mm. 

 embryo, a condition is met with such as is shown in figure 9. Specimens at this 

 early period, when fixed in formalin, are moderately transparent, and thus it is 

 difficult to make out their true form. By slightly staining the specimen, as was 

 done in this case, it is possible to distinguish more clearly the surface modeling 

 and to represent these structures accurately. In order to display completely the 

 face region, the greater part of the trunk was removed, leaving only the pericardial 

 dome and cut end of the aortic trunk. 



The drawing which we are considering was made directly from the specimen 

 and presents a three-quarter view of the four branchial arches of the left side. It 

 is only by tracing backward from older stages, where the auricular area is pro- 

 nounced, that one can outline it at this stage. For this purpose actual specimens 

 were compared, as were also enlarged models in which the branchial region was 

 completely exposed and in which analogous parts could be identified. When the 

 same proportionate area is plotted in this way on the mandibular and hyoid arches, 

 one obtains the result shown in blue in figure 9. Practically the whole surface of 

 the hyoid arch subsequently takes part in three thickenings, known as the auricular 

 hillocks numbers 4 to 6. In the same way the greater part of the surface of the 

 mandibular arch enters into the formation of the first three hillocks. It is of 

 interest to note how closely the auricular areas of the right and left sides approach 

 each other in the midventral line. It is from the small interval between them that 

 the mandible and its associated soft parts must be derived. It is true, there was 

 some difficulty in determining the boundary line between the auricular area and 

 the midventral segment of the mandibular arch, as the line of junction is not char- 

 acterized by any surface marking, nor can any histological difference be yet recog- 

 nized in serial sections. The area as outlined, however, agrees in form with that 

 seen in the next older stage and is probably accurate. 



When the topography of the auricular region in the stage shown in figure 9 

 is considered, it can be readily understood that failure on the part of the mandible 

 to develop would leave the external ears near the median line in front of the upper 

 part of the neck. The literature records cases of agnathia or synotia which are of 

 this nature. Two of these are reproduced in text-figures 3 and 4. In them the 

 early position of the auricles is retained, owing to the fact that there was nothing to 

 wedge the two auricles apart, as is normally done by the growing mandible. 



