DEVELOPMENT OF THE NERVOUS SYSTEM. 



55 



slit and then grows backwards until a series of ganglia are formed, 

 one over each gill slit. In bony fishes, amphibia and higher 

 forms these ganglia tend to become consoHdated and are not 

 symmetrically placed over the gill slits. Where the gangHa 

 reach the ectoderm over the gill slits the ectoderm becomes thick- 

 ened by multiplication of its cells and at least in some lower 

 vertebrates, cells proliferate from the ectoderm and join the 

 ganglia. The thickening of ectoderm is called the epibranchial 

 placode. Each ganglion is now composed of two parts, a median 

 part derived from the neural crest and a lateral part derived from 

 the epibranchial placode. The ganghon may now be called 

 the epibranchial ganglion. In man the ganglion petrosum on the 

 IX nerve and the ganglion nodosum on the X nerve seem to repre- 



Sens.organ on gang.petros. 



Fig. 31. — Reconstruction of the peripheral nerves in a four weeks human 

 embryo, 7. o mm. long. Enlarged 16.7 diameters. From Streeter. 



sent the epibranchial ganglia of fishes, and ectodermal placodes 

 are described in connection with these ganglia in man (Fig. 31), 

 but no migration of cells from the ectoderm to join the ganglia 

 has been made out. From each ganglion so formed the ramus 

 posttrematicus described in the previous chapter grows downward 

 behind the gill slit and the ramus pharyngeus grows forward and 

 inward to the roof of the pharynx. From the ramus pharyngeus 

 in forms above the cyclostomes the ramus praetrematicus goes 



