THE EARLY DEVELOPMENT OF THE MAMMAL 413 



uniting with the maxillary processes of the first visceral arch 

 to form the rudiments of the upper jaw and lip. The man- 

 dibular processes are still separated medially by a groove. 

 During the sixth week the eyes become visible from in front, 



FIG. 172. The development of the face of the human embryo. After Retzius. 

 A. 18 mm. embryo, X4. B. 25 mm. embryo, X4. C. 42.5 mm. embryo, X2. 

 D. 117 mm. embryo, X4/5. For explanation see text. 



the upper lip begins to enlarge though still indented medially, 

 the mandibular processes fuse completely forming the com- 

 pleted rudiment of the lower lip and jaw, and the chin appears. 

 The medial olfactory processes (globular processes) soon fuse 

 together forming the nasal septum (Fig. 172, A), and the nose 

 becomes slightly marked off from the forehead by a groove. 



