372 



THE PERIPHERAL NERVOUS SYSTEM 



of 6 to 7 mm. In 8 mm. embryos the invagination has produced a distinct 

 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 (p. 146) that each first branchial arch 

 forks into a maxillary and mandibular process. Dorsal to the oral 

 cavity is the fronto-nasal process of the head, lateral to it the maxillary 

 processes, and ventral to it are the mandibular processes (Fig. 97). With 



Fore-brain 



Vomero-nasal organ 



Olfactory placode 



Olfactory placode 



Olfactory fossa 

 E 



Median nasal process 



Nasal fossa 



Telencephalon 

 Nasal fossa 



Lot. nasal process 

 Med, nasal process 

 Maxillary process 



Epithelial plate 



FIG. 369. Sections through the olfactory anlages of human embryos. A, 4.9 mm. (X 20); 

 B, 6.5 mm. (X 13); C, 8.8 mm. (X 13); D and E, 10 mm. (A, B and Cfrom Keibel and Elze). 



the development of the nasal pits, the fronto-nasal process is divided into 

 paired lateral nasal processes and a single median frontal process, from which 

 are differentiated later the median nasal processes, or processus globulares 

 (Fig- 37). 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. 3 70 ,4) . 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. 370). 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. 369 D, E). 



