THE EAELY NEKVOUS SYSTEM. 



31 



As the lateral margins of the neural plate are raised the plate is necessarily folded 

 longitudinally, and the- sulcus so formed is called the neural groove. Each side 

 wall of the neural groove, formed by the corresponding half of the neural plate, is 

 a neural fold. At a very early period the neural folds unite anteriorly to form the^ 

 anterior boundary of the neural groove, and, somewhat later, they unite posteriorly,* 

 caudal to the neurenteric canal and across the anterior end of the primitive streak. 

 After the lateral boundaries and the anterior and posterior extremities of the neural 

 groove are defined, the lateral margins of the neural folds converge until they meet 

 and fuse in the median plane, and the neural groove is thus converted into the 

 neural tube, which possesses a floor or ventral wall, formed by the central part of 



Neural crest 



Primitive ganglion 



Surface ectoderm 



^Floor- plate - 

 (1) 



Roof-plate 



Ependyma cells 

 'osterior nerve-root 



Posterior 

 nerve-root 



Anterior 

 nerve-root 

 Sympathetic 

 ganglion 



Chromamn cells 



Basal lamina with 

 neuroblasts 

 (3) 



Roof-plate 



Surface ectoderm 



Spinal 

 ganglion 



Sympathetic 

 ganglion 



Chromamn cells 



Central canal 



Ependyma cells 

 Mantle layer 



Peripheral 

 layer 



Gut 

 Anteriorlnerve-root 



Sympathetic ganglion - 



Chromamn cells 



Gut 



Roots of 

 sympathetic 

 ganglion 

 Sympathetic nerve 



(4) Secondary sympathetic ganglion 



FIG. 44. DIAGRAMS illustrating the formation of (1) the rudiments of the primitive ganglion from the neural 

 crest. (2) The differentiation of different parts of the primitive ganglion into permanent ganglion root, 

 sympathetic ganglion, and masses of chromaffin cells. (3) The formation of the anterior and posterior 

 nerve-roots. (4) The differentiation of the walls of the neural tube into ependymal matter and 

 peripheral layers. 



The cells of the primitive ganglion which form the primitive sheaths of the nerves are not shown in the 

 diagrams. 



the original neural plate and called the basal plate or floor-plate ; a dorsal wall or 

 roof-plate, and two lateral walls formed by the lateral parts of the neural plate. 



The fusion of the lateral margins of the neural plate to form the roof-plate 

 of the neural tube commences in the cervical region, and from there extends 

 cranialwards and caudalwards, therefore the last parts of the roof-plate which 

 are formed are its anterior and its posterior extremities ; consequently, for a time, 

 the neural canal, which is the cavity of the tube, opens on the surface at its 

 anterior and posterior ends; the anterior opening being called the anterior 

 neuropore, whilst the open part at the posterior end is termed the posterior 

 neuropore (Fig. 43). Eventually, however, about the third week of embryonic 



