THE CEKKBRAL NEK\ I S 



365 



In aquatic vertebrates, special somatic sensory fibers from the lateral line organs 

 join the facial, glossopharyngeal, and vagus nerves, and their ganglion cells form part 

 of the geniculatc, petrosal, and nodose ganglia. In human embryos the organs of the 

 lateral line are represented by ectodermal thickenings, or placodes, which occur tempo- 



Cong, jugulare N. 10 Accessory root ganglia 



Gang, superius N. 9 



Gang, petrosum N. 9 

 N. lympanicus 



Br. to carotid plexus 

 N. glossopharyngeal 



Gang. Froriep 



N. accessorius 



N. hypoglossu, 



N. laryng. sup. 

 Gang, nodosum N. 10 



Sympathetic trunk 



FIG. 365. A reconstruction of the peripheral nerves in an embryo of 17.5 mm. (Streeter). 



X 16.7. 



rarily over these ganglia. The nervous elements supplying these vestigial organs have 

 completely disappeared. 



Segmentation of the Vertebrate Head. The vertebrate head undoubtedly consists 

 of fused segments. This was suggested to the earlier workers by the arrangement of 

 the branchial arches (branchiomerism) , and by the discovery, in the embryos of lower ver- 

 tebrates, of so-called head cavities, homologous with mesodermal segments. (Note also 

 the presence of neuromeres, p. 334.) 



