THE LATER DEVELOPMENT OF THE CHICK 309 



the scope of this work. (Further information on this general 

 subject is found, together with references to the literature, 

 in Johnston's " Nervous System of the Vertebrates/' Philadel- 

 phia, 1906.) 



2. The Cranial and Spinal Ganglia 



These structures are derived from the neural crests, whose 

 formation has already been described. The neural crests 

 finally extend the entire length of the nervous system. In 

 the trunk region they begin to extend laterally between the 

 ectoderm and the mesodermal somites. In the intervals 

 between successive somites the neural crest becomes inter- 

 rupted, the cells apparently, though not certainly, being 

 converted into mesenchyme. By this process each crest is 

 broken into a metameric series of cell-groups. These groups 

 are the rudiments of the spinal ganglia, and therefore also of 

 the dorsal (afferent) roots of the spinal nerves. Each group 

 contains neuroblasts as well as indifferent cells, and very 

 early comes into close relation with the cells of the adjoining 

 somite. 



In the region of the somites of the head the neural crests are 

 poorly developed, in correlation with the absence of ganglia in 

 these segments. But throughout the remainder of the head 

 they are well developed and form the rudiments of the ganglia 

 and certain of the branches of the cranial nerves. As in the 

 trunk region, the continuous crests are broken up by the con- 

 version of certain regions into mesenchyme. The regions re- 

 maining form the rudiments of the ganglia of the X, IX, VIII, 

 VII, and V cranial nerves. The ganglia of the IX and X 

 nerves arise together and are divided, toward the close of the 

 second day into separate cell-groups by the formation of the 

 gill-pouches of the region (Fig. 130). Thus the ganglion of the 

 IX nerve, ganglion petrosum, is left just above the third visceral 

 arch, and that of the X nerve, ganglion jiigulare-nodosum, above 

 the fourth and fifth visceral arches. Each of these acquires a 

 connection with an ectodermal branchial sense organ, or 



