THE NERVOUS SYSTEM 541 



the neural plate. Next to the notochord, the nervous system is the first 

 organ to develop. By the elevation of its edges, the neural plate is 

 converted into a neural groove bordered by neural folds. The anterior 

 more widely expanded portion forms the brain, and the narrower posterior 



Fig. 446. — The sympathetic system in a i6-mm. human embryo. The ganglionated 

 trunk is heavily shaded. The first and last cervical, thoracic, lumbar, sacral and coccy- 

 geal spinal ganglia are numbered, a, Aorta; ace, accessory nerve; car, carotid artery; 

 cil, ciliary ganglion; coe, coeliac artery; Ht, heart; nod, nodose ganglion; ot, otic ganglion; 

 pel, petrosal ganglion; s-m, submaxillary ganglion; s.mes. superior mesenteric artery; 

 sph-p, sphenopalatine ganglion; spl, splanchnic nerve; st, stomach. (From Bremer's 

 "Text Book of Histology," after Streeter.) 



portion the spinal cord. The transition between the two is, however, in 

 most animals, gradual rather than abrupt. 



As the neural folds rise, they bend towards the median plane and 

 finally unite to form a neural tube with an anterior enlarged brain and a 

 posterior constricted spinal cord. The closure of the neural tube begins 

 in the neck region and proceeds craniad and caudad. But even before 

 the neural folds in the cephalic region unite, a series of three expansions 



