826 



GENERATION. 



remains by the side, which forms, externally, the external auditory meatus, 

 and internally, 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 projections 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 second- 

 ary projections, which leave certain 

 permanent openings, forming the 

 mouth, nose etc. 



In the very first stages of develop- 

 ment of the head there is no appear- 

 ance of the face. The cephalic extrem- 

 ity consists simply of the cerebral vesi- 

 cles, the surface of this enlarged por- 

 tion of the embryon being covered, in 

 front as well as behind, by the epiblast. 

 During the sixth week, after the cavity 

 of the pharynx has appeared, the mem- 

 brane 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 

 appears as a projection of the meso- 

 blast, extending forward. This is soon 

 marked by two secondary projections, 



the upper projection forming the superior maxillary 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 lip. 

 At the same time there is a projection from above, extending between the 

 two superior projections, which is called the frontal, or intermaxillary pro- 

 cess. This extends from the forehead that portion which covers the front 

 of the cerebrum downward. The superior maxillary projections then ad- 

 vance forward, gradually passing to meet the frontal process, but leaving two 

 small openings on either side of the median line, which are the openings of 

 the nostrils. The upper portion of the frontal process thus forms the nose ; 

 but below, is the lower end of this process, which is at first split in the medi- 

 an line, projects below the nose, and forms the incisor process, at the lower 

 border of which are finally developed the incisor teeth. As the superior max- 

 illary processes advance forward, the eyes are moved, as it were, from the 



S 



FIG. 307. Mouth of a human embryon of tioen- 

 ty-five to twenty-eight days; magnified 15 

 diameters (Coste). 



1, median or frontal process, the inferior portion 

 of which is considerably enlarged ; 2, right 

 nostril : 3, left nostril ; 4, 4, interior maxil- 

 lary processes, already united in the median 

 line ; 5, 5, superior maxillary processes, 

 which have become quite prominent and 

 have descended to the level of the slope of 

 the frontal process : 6, mouth ; 7, first vis- 

 ceral arch : 8, second visceral arch ; 9, third 

 visceral arch ; 10, eye ; 11, ear. 



