THE DEVELOPMENT OF THE NERVOUS SYSTEM 



591 



contact through a telodendrion either directly or indirectly (through asso- 

 ciation neurons) with the dendrons of motor neurons. Its peripheral 

 process is to be regarded as the dendron, modified structurally so as to be 

 essentially identical with the axon. 



The sympathetic may be regarded as a cenogenetic addition to the 

 cerebrospinal nervous system. Ontogenetically it appears in man at tho 



FIG. 512. SECTION OF WALL OP FORE- 

 BRAIN OF FOUR-DAY CHICK EMBRYO. 



A, apolar neuroblast; B, bipolar neuro- 

 blast; a, the beginning of the sprouting of 

 the axon; c, axons, showing the terminally 

 expanding growth area ('cone of growth'). 

 Cajal's silver technic. (Heidenhain, after 

 Cajal.) 



FIG. 513. DIAGRAM OF 

 A TRANSECTION OF 

 THE SPINAL CORD OF 

 AN EARLY EMBRYO, 

 SHOWING THE MIGRA- 

 TION OF NEUROBLASTS 

 TOWARD THE MARGI- 

 NAL VEIL, AND THE 

 VENTRAL NERVE 

 ROOT. 



o, neural canal; b, ven- 

 tral root. (After His.) 



beginning of the fifth week (7 mm. embryo). Phylogenetically, it first 

 appears in cyclostomes. Kuntz (Jour. Comp. Neur. and Psyc., 29, 3, 1910) 

 traces the neurons of the sympathetic trunks and the prevertebral plexuses 

 to neuroblasts in both the neural crest and the medullary wall, from whence 

 they migrate by both the dorsal and ventral nerve roots. The neurolemma 

 cells, as also the capsular elements of the sympathetic neurons, arise from 

 'indifferent' cells which migrate from these same locations and proliferate 

 and differentiate along their course. The cardiac plexus and the sympa- 

 thetic plexuses in the walls of the visceral organs (terminal plexuses ; 'vagal 

 sympathetic' plexuses Kuntz) have their origin, according to Kuntz, in 

 nervous elements which migrate from the hind-brain and the vagus gan- 



