126 DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



stages, and we can now speak of a distinct fossa angularis. The ventral third of 

 this fossa becomes relatively deeper to form the external auditory meatus, while the 

 remainder is eventually taken up in the formation of the auricle. 



In embryos 16 to 18 mm. long the relations are such as are shown in figure 12, 

 which is a drawing of a model posed similarly to those shown in figures 9 and 11. 

 Owing to the foreshortening in a three-quarter view of this kind, one is apt to get a 

 false impression as to the height of the head; in a true profile view the distance 

 between the eye or ear and the dorsal midline over the midbrain or cerebellum 

 would be much greater. At this time the auricular hillocks, except those continued 

 as the tragus and antitragus, have lost their identity and have been molded into 

 the early form of the definitive parts of the auricle. The beginning helix can be 

 definitely outlined, and less distinctly the crus, the former being entirely a deriva- 

 tive of the hyoid bar, the latter a derivative of the mandibular bar. A fact of 

 interest is that, whereas the crus and tragus form a relatively small part of those 

 adult surface structures that are derived from the mandibular bar, the scapha- 

 helix and antitragus (eventually, also, the anthelix and lobule) constitute the only 

 permanent surface representatives of the hyoid bar. 



With the topography of the auricular area thus identified in the four stages 

 just represented, a comparison of these stages discloses certain general facts. 

 Only two gill-bars take any prominent part in the formation of the surface struc- 

 tures of the lower jaw. Of these, the first or mandibular bar contributes by far 

 the greater amount; the second or hyoid bar supplying only a portion of the auricle. 

 The third and fourth bars have no permanent surface record of their existence. 

 The auricular area, relative to the size of the head, covers at first a large surface, 

 but as we pass from simple gill-bars to the stage of hillocks and then to the definite 

 auricle it becomes progressively smaller. Were we to trace it to the stage of 20 to 

 30 mm., when the face is more fully formed, we would find it still smaller. There- 

 after, the increased growth and spreading character of the free auricle counteract 

 the previous relative decrease in size. 



Another and perhaps the most conspicuous feature in the topography of the 

 developing auricle is its lateral and dorsal migration. In the stage of simple gill- 

 bars the two auricular areas nearly meet in the midventral line, but, as can be seen in 

 figures 9 to 12, they are gradually crowded sidewise coincidentally with the develop- 

 ment of the mandibular apparatus and the structures at the base of the skull. 

 A true profile of figure 12 would show the auricle higher on the side of the head than 

 it there appears. It is to be remembered that this migration is relative rather than 

 real. At all stages the mouth line is in a plane roughly intersecting the middle 

 of the auricle ; the appearance of an upward migration is due chiefly to the growth 

 of the angle of the jaw and the elongation of the neck. 



THE BRANCHIAL HILLOCKS. 



In embryos 4 to 6 mm. long the mandibular and hyoid bars are each subdivided 

 by a transverse groove into a dorsal and a ventral part, as can be seen in figures 

 13 and 14 (plate 2); also figure 9 (plate 1). These are not to be confused with the 



