148 TEXT-BOOK OF EMBRYOLOGY. 



lary processes, and to a slight extent by the mandibular processes. Between 

 the maxillary and mandibular processes on each side a notch marks the angle 

 of the mouth. 



As development proceeds these structures become more elaborate and enter 

 into more intimate relations with one another. The naso-frontal process 

 extends farther downward toward the mandibular processes, so that the 

 oral fossa becomes more nearly enclosed and the entrance to it reduced to a 

 crescent-shaped slit the mouth slit. At the same time two secondary processes 

 develop on each side from the naso-frontal process. One of these the 

 medial nasal process forms near the medial line; the other the lateral nasal 

 process forms more laterally (Figs. 135, 136). Between the two processes there 



Cerebral hemisphere 



Lat. nasal process 

 Nasal pit 



Med. nasal process ^^^Bi^^kfl&JB Naso-optic furrow 

 Angle of mouth M^ || Maxillary process 



Mandibular 



FIG. 134. Ventral view of head of 8 mm. human embryo. His. 



is a depression the nasal pit which marks the entrance to the future nasal 

 cavity. The maxillary process on each side grows farther toward the medial 

 line and comes in contact with the lateral and medial nasal processes. 



At this stage all the elements which enter into the fundamental structure of 

 the face region are present. Further development consists essentially of 

 fusions between these various elements. 



The two medial nasal processes come closer together to form the single 

 medial process which gives rise to the medial portion of the upper lip and to the 

 adjoining portion of the nasal septum. The maxillary process on each side 

 fuses with the corresponding lateral and medial nasal processes. This 

 fusion obliterates the naso-optic furrow and also shuts off the communi- 

 cation between the mouth slit and the nasal pit (Figs. 136, 137). The lateral 

 nasal process gives rise to the wing of the nose; the maxillary process gives rise 



