154 



THE DISSECTION OF PIG EMBRYOS FOR STUDY 



and most prominent at 12 mm. (Fig. 144 A). Soon, in 13 to 14 mm. embryos, 

 the median nasal processes fuse with the maxillary processes of the first arch and 

 constitute the upper jaw (Fig. 144 B). The lateral nasal processes fuse with the 

 maxillary processes and form the cheeks, the lateral part of the lips and the alae 

 of the nose. Later, the median nasal processes unite and become the median 

 part of the upper lip and the columna nasi. 



The early development of the face is practically the same in human embryos 



Lat. nasal process 

 Olfactory pit 

 Med. nasal process 



Mandible 



Br. archil 

 Ventral aorta 



Eye 



Lacrymal groove 



Maxillary proc. 



Br. cleft. I 

 Br. cleft II 



Lat. nasal process 



Maxillary process 

 . Mandible 

 Br. cleft 



Ext. naris 



Eye 



Med. nasal process 



Oral cavity 



Ext. ear 



FIG. 144. Two stages showing the development of the face in pig embryos. A , Ventral view of face of 

 a 12 mm. embryo; B, of a 14 mm. embryo. 



(Fig. 145). At the end of the fourth week, the lateral and median nasal processes 

 have developed. During the sixth week, the maxillary processes fuse with the 

 nasal processes, and at the end of the second month the median nasal processes 

 have united. The mandibular processes fuse at the sixth week and from them a 

 median projection is developed which forms the anlage of the chin. 



The lips begin to appear as folds at the sixth week. As the median nasal processes and 

 the maxillary processes take part in their development, the failure of these parts to fuse may 

 produce hare lip. The lips of the new-born child are peculiar in that their proximal surfaces 

 are covered with numerous villi, finger-like processes which may be a millimeter or more in 

 length. 



