160 



EMBRYOLOGY 



V, 



NEURAL FOLDS 



NOTOCHORD 



NEURAL PLATE 



CENTRAL CANAL 



•NEURAL CREST 



B 



Fig. 94. The formation and de- 

 velopment of the neural tube. In A 

 the future spinal cord and spinal 

 ganglia are simply a plate of elon- 

 gated cells. This plate folds to form 

 a tube, as in B. When the folds fuse 

 a strip of tissue is pinched off on 

 either side and this tissue is called 

 the neural crest. In C the neural tube 

 is shown with a central canal, while 

 the neural-crest cells migrate to form 

 the sympathetic ganglia and also the 

 deeper lying visceral ganglia. Some 

 of the crest cells migrate still fur- 

 ther and form the cortex of the 

 adrenal gland. Other crest cells mi- 

 grate into the feather germ and de- 

 posit pigment. Still others migrate 

 along the nerve fiber and deposit the 

 myelin sheath. Finally, some of the 

 crest cells send out cones of growth 

 and thus become the sensory nerves 

 from the spinal ganglia (D). The 

 neuroblasts in the neural crest send 

 out one outgrowth which migrates 

 into the spinal cord dorsal root and 

 another that migrates out into the 

 surrounding tissues to form a sen- 

 sory nerve fiber. 



The neuroblasts within the cord 

 (D) send outgrowths through the 

 ventral root to muscles and also out- 

 growths which come in close prox- 

 imity to the endings of the sensory 



nerves. Other neuroblasts within the cord differentiate into neurons which connect one 



side of the cord with the other, thus forming a commissure. 



VISCERAL GANGLION 



NEURAL CREST 



SYMPATHETIC 

 GANGLIA 



CORTEX OF ADRENAL 



DORSAL ROOT 



SPINAL GANGLION 

 SHEATH CELLS 



COMMISSURE 



SYMPATHETIC GANGLION 



and form the deep-lying ganglia of the viscera. Other cells migrate to the 

 region of the Wolffian duct, where they fuse with mesoderm cells to form 

 the adrenal gland. Still other cells migrate to a region near the spinal cord 

 and develop into the sympathetic ganglia. 



Another group of neural-crest cells migrates to the limb and enters the 

 feather germs of the skin. This migration has been demonstrated clearly by 

 grafting the neural crests from a species X, which is colored, to the embryo 

 of a species Y, which is white. Some of the feathers of Y will then be colored. 



