37 



THE PERIPHERAL NERVOUS SYSTEM 



rounding ectoderm as ventro-lateral thickenings near the tip of the head. They 

 are flattened and begin to invaginate in embryos of 6 to 7 mm. In 8 mm. embryos 

 the invagination has produced a distinct pit or fossa surrounded everywhere save 

 ventrally by a marginal swelling. 



The later development of the olfactory organ is associated with that of the 

 face. It will be remembered (see p. 153) that the first branchial arch forks into 

 the maxillary and mandibular processes. Dorsal to the oral cavity is the frontal 

 process of the head, lateral to it the maxillary processes, and ventral to it are the 

 mandibular processes (Fig. 144). With the development of the nasal pits the 



Nasal septum 



Exi.naris 

 Lat. nasal process 



Ext.nar'is 



lot. nasal 

 process 



Med. nasal 

 process 



Maxillary 



0ra1cavity 



Oral cav//~y 



Mandible 



Med. nasal process 

 A B 



FIG. 351. Two stages in the development of the jaws and nose. A , ventral view of the end of the head 

 of a 10.5 mm. embryo (after Peter); B, of an 11.3 mm. embryo (after Rabl). 



frontal process is divided into paired lateral nasal processes and a single median 

 frontal process, from which later are differentiated the median nasal processes, or 

 processus globular es (Fig. 351). The nasal pits are at first grooves, each bounded 

 mesially by the median frontal process and laterally by the lateral nasal process 

 and the maxillary process (Fig. 35 1 A) . The fusion of the maxillary processes with 

 the ventro-lateral ends of the median frontal process converts the nasal grooves 

 into blind pits or fossae, shutting them off from the mouth cavity (Fig. 351 A, B). 

 Thus in embryos of 10 to 12 mm. the nasal fossa has but one opening, the external 

 naris, and is separated from the mouth cavity by an ectodermal plate (Fig. 350 

 D,E). 



