122 PROCEEDINGS OP THE AMERICAN ACADEMY 



straight, it is now beut near the middle by an elevation formed on the 

 anterior margin of the hyoid arch (Fig. 2,pe), which projects over 

 the cleft. This elevation overlaps a depressed portion of the mandibu- 

 lar arch, and plays an important part in the development of the am-icle 

 of the ear. 



In an embryo 9 mm. long (Fig. 3) the cleft has become more bent 

 by the forward growth of the elevation of the hyoid arch, and the deep- 

 ening of the corresponding depression of the mandibular arch. The 

 furrow of Hunt has become limited to a small triangular depression, 

 the ventrally directed apex — the part corresponding to the dorsal end 

 of the cleft — being the deepest. At this stage the depression is quite 

 conspicuous, since the roots of the hyoid and mandibular arches, which 

 form its ventral walls, are much swollen. The elevation on the hyoid 

 (¥ig. 3, p e) has become more prominent, and has thus affected the 

 shape of the first arch, the mandibular branch of which, as seen in 

 profile, has taken an almost rectangular outline. 



With embryos 10 mm. long (Fig. 4) the cleft is still more bent, and 

 is sharply limited by the hyoid wall. Its anterior margin, however, is 

 not so sharply marked, since the mandibular wall gradually slopes into 

 the cleft. At either side of the hyoid elevation the cleft is deepest, 

 becoming more shallow as one approaches its extremities. The eleva- 

 tion on the hyoid has increased considerably in size, and at either side 

 of it is formed a secondary elevation (s d, s v). Each of these, although 

 very slight as yet, is clearly distinguishable, and is separated from the 

 primary elevation by a shallow depression (d). On the mandibular 

 arch opposite these elevations there are others {mde,mve) which 

 form the angles of that arch : they give a definite limitation to the 

 cleft, and at the same time clearly define the rudimentary jaw. The 

 depression of Hunt has become less pronounced because of the outward 

 growth of the tissue forming its floor, owing to the more complete 

 fusion of the roots of the mandibular and hyoid arches. 



In embryos 11 mm. long (Fig. 5) the primary elevation of the hyoid 

 has assumed a conical shape ; it is thickened and swollen at the base, 

 but thinned out at the apex, and is very sharply limited. Tiie second- 

 ary elevations have become distinctly separated from the primary by 

 the deepening of the depressions between them and the latter, and have 

 thus become much more pi-ominent. At either side of the primary ele- 

 vation of the hyoid the cleft has the appearance of a deep hole, which 

 is sharply bounded by the almost perpendicular walls of the hyoid and 

 mandibular arches. The elevation at the root of the mandibular arch 

 is still prominent, whereas the depression of Hunt is almost obliterated, 

 at least in embryos prepared by the picro-sulphuric acid process. 



