Development of the Third Day 195 



that are derived from it. The main branch 

 of this arch, on each side, meets its fellow 

 of the opposite side in the mid-ventral line, 

 and fuses to form the basis of the mandible 

 or lower jaw ; hence its name, the mandibular 

 arch (Fig. 64, MN). From the anterior edge 

 of the dorsal end of each half of the man- 

 dibular arch a small branch grows forward 

 and downward, during the fourth or fifth 

 day (Fig. 64, MX), towards a triangular 

 median process from the front of the head. 

 This median process has already been men- 

 tioned, and is named, from the region of 



. the head formed by it, the fronto-nasal process 

 (Fig. 64, FP\ The branches or processes 

 from the mandibular arch are the maxillary 

 processes, and from them the upper half of the 



, jaw is formed. The two maxillary processes 

 do not meet each other in the middle line, 

 as do the two parts of the mandibular arch, 

 but fuse with each side of the median fronto- 

 nasal process, to form the upper half of the 

 jaws. 



The abnormality known as harelip, some- 

 times seen in human beings, is caused by 

 the failure of one of the maxillary processes 

 to fuse completely with the fronto-nasal 



