OF ARTS AND SCIENCES. 123 



A series of embryos from 11 mm. to 17 mm. long shows the gradual 

 disappearance of the elevation at the root of the mandible and of the 

 depression of Hunt. The primary elevation of the hyoid arch (Fig. 6) 

 becomes more and more developed, and its growth is attended with 

 apical attenuation. The secondary elevations become less conspicuous, 

 and show a tendency to coalesce with the primary. The superior (dor- 

 sal) one, however, is still distinct. The mandibular wall continues to 

 slope rather gradually into the cleft. The latter is now confined to the 

 side of the head, and the surrounding parts show some resemblance to 

 the permanent external ear; the apex of the primary elevation of the 

 hyoid arch corresponds to the tip of the auricle. 



In the case of an embryo 19 mm. long (Fig. 7), the walls of the 

 meatus (for such we may now call it) are steep and sharply limited. 

 The inferior (ventral) secondary elevation of the hyoid has disappeared, 

 whereas the superior one still exists. 



When the embryos have attained the length of 25 mm. (Fig. 8) the 

 auricle has a triangular shape, thus approximating its ultimate form. 

 The superior secondary elevation has by this time disappeared, and the 

 whole aspect now closely resembles that of the ear of the animal at birth. 



From this description it will be seen that the development of the 

 auricle is the most salient feature of the external changes in the walls 

 of the cleft, and is distinguishable at a much earlier epoch than has 

 hitherto been claimed. It is to be seen first in an embryo 8 mm. long 

 as a slight elevation of the anterior margin of the hyoid arch (Fig. 2, 

 p e). This elevation gradually increases in prominence, but always 

 preserves its primitive relation to the cleft, since it continues to point 

 directly across the centre of the latter. In an embryo 25 mm. long 

 the centre of the meatus is directly beneath the apex of this auricular 

 bud (Fig. 8). Inasmuch as the centre of the cleft of an 8 mm. em- 

 bryo occupies the same position in relation to the incipient auricle, it 

 is fair to infer that the external meatus occupies the region of the 

 centre of the cleft. 



The long diameter of the slit does not increase to any great extent 

 until the embryo is 30 mm. or 40 mm. long, although it increases rap- 

 idly in width after the embryo attains a length of 17 mm. From this 

 it would seem that the depression described by Hunt (which lies at the 

 dorsal extremity of the cleft, and owes its existence to the less rapid 

 growth of the tissue at that point), instead of deepening, becomes ob- 

 literated by a thickening of the tissue which forms its floor. This 

 thickening constitutes the wall bounding the dorsal extremity of the 

 cleft. lu other words, the inclined floor of this depression forms the 



