THE SPINAL CORD AND BRAIN 



809 



severs all connection with the ectoderm from which it developed; but for a brief 

 period the continuity is preserved in an af.enuated septal mass, the neural crest 

 (Fig. 577). The cell elements of this crest subsequently become detached from 

 the superficial ectoderm, the continuity of which is again restored to form the 

 integument. They then pass ventrad to either side of the neural tube, prolifer- 



NEURAL PLATE 



NEURAL GROOVE 



SOMATOPLEURE- 



SPLANCHNOPLEURE 



PARAXIAL 

 MESODERM 



ENTOOERM 



FIG. 577. Diagrams showing development of neural tube and crest. 



ate by mitosis, and accumulate in paired masses, corresponding in number to^ 

 the segments of the body, to become, in part at least, the cerebrospinal ganglion 

 cells of the afferent system, while other similarly paired masses migrate farther 

 ventrad to a prevertebral position to form the gangliated cord and widely spread 

 plexuses of the sympathetic system. From the tissues of the wall of the neural 



NEURAL TUBE 



SPINAL GANGLION 



VENTRAL ROOT 



RAMUS COM- 

 MUNICANS 



SPINAL NERVE 



.SYMPATHETIC 

 GANGLION 



MESONEPHROS 



MESENTERY 



FIG. 578. Diagram showing development of a spinal nerve and its components, together with the 



spinal and sympathetic ganglia. 



tube and its temporary crest the entire nerve system of complex and intricate 

 structure is developed. The cavity of the tube shares in the developmental 

 growth changes to become the ventricular system of the brain and central canal 

 of the spinal cord. The major details of the development of the principal divisions 

 will be considered in appropriate chapters. 



