372 



THE PERIPHERAL NERVOUS SYSTEM 



In embryos of 4 to 5 mm. (Fig. 369) the placodes are sharply marked ofE from 

 the surrounding ectoderm as ventro-lateral thickenings near the top 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 every- 

 where save ventrally by a marginal swelKng. 



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

 face. It will be remembered (cf. p. 145) that the first branchial arch forks into 

 the maxillary and mandibular processes. Dorsal to the oral cavity is the fronto- 



Fore-brain 



Vomero-nasal organ 



Olfactory placode 



Olfactory placode 



Olfactory fossa 

 E 



Median nasal process 



Nasal fossa 



Telencephalon 



■Nasal fossa 



Lat. 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 C from Kiebel and Elze.) 



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

 are the mandibular processes (Fig. 97) . With 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 proc- 

 esses, or processus globulares (Fig. 370). 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. 370 A). The fusion of the maxillary 

 processes with the ventro-lateral ends of the median frontal process converts the 



