412 GENERATION. 



forms the lesser cornua of the hyoid bone, the stapes, and the 

 styloid ligament ; the third arch forms the body and the 

 greater cornua of the hyoid ; and the fourth arch forms the 

 larynx. The first cleft, situated between the first and the 

 second arch, becomes obliterated in front by a deposition of 

 plastic matter, but an opening remains by the side, which 

 forms, externally, the external auditory meatus, and inter- 

 nally, the tympanic cavity and the Eustachian tube. The 

 other clefts become obliterated as the arches advance in their 

 development. 



From the above sketch, it is seen that the face and the 

 neck are formed by the advance and closure in front of pro- 

 jections from behind, in the same way as the cavities of the 

 thorax and abdomen are closed ; but the closure of the first 

 visceral arch is complicated by the projection, from above 

 downward, of the frontal, or intermaxillary process, and by 

 the formation of several secondary projections, which leave 

 certain permanent openings, forming the mouth, nose, etc. 

 These processes of development, we shall now attempt to 

 follow. 



In the very first stages of development of the head, there 

 is no appearance of the face. The cephalic extremity consists 

 simply of the cerebral vesicles, the surface of this enlarged 

 portion of the embryon being covered, in front, as well as 

 behind, by the external blastodermic membrane. During 

 the sixth week, after the cavity of the pharynx has appeared, 

 the membrane gives way in front, forming a large opening, 

 which may be called the first opening of the mouth. At this 

 time, however, the face is entirely open in front as far back 

 as the ears. The first, or the superior visceral arch, now ap- 

 pears as a projection of the middle blastodermic layer, ex- 

 tending forward. This is soon marked by two secondary 

 projections, the upper projection forming the superior maxil- 

 lary portion of the face, and the lower, the inferior maxilla. 

 The two projections which form the lower jaw soon meet 

 in the median line, and their superior margin is the lower 



