CHAPTER VIII. 

 EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



The brain and spinal cord are developed from the neural tube; 

 the spinal and sympathetic ganglia and the ganglia of sensory 

 cerebral nerves are derivatives of the neural crest (Figs. 16 and 

 114). 



Very soon after conception there appears along the median 

 line, in the back of the embryo, a plate of epiblast, called the 

 medullary plate. That plate soon presents two longitudinal eleva- 

 tions, the medullary ridges, separated by a median furrow, the 

 neural groove (Fig. 16). At first the medullary plate is formed of 

 a single layer of columnar cells; but those cells undergo rapid 

 mitosis, as the medullary ridges are developed, and arrange them- 

 selves in several layers which thus bound the neural groove (Fig. 

 114). The infolding of the medullary ridges, together with their 

 rapid growth, roofs over and closes hi the neural groove and pro- 

 duces the neural tube (Fig. 16). The tube is first formed in the 

 cervical region, whence the fusion of the medullary ridges extends 

 in both 'directions. By the approximation of the medullary ridges 

 a slight paramedian crest is produced on either side,, called the 

 neural crest. The neural tube and the neural crest are nearly 

 complete by the fifteenth day after conception, the groove is open 

 only at the position of the future median aperture of Magendie. 

 Both ends of the tube are open for a short time, the anterior 

 closing first. The mesoblast which later grows around the 

 neural tube is developed into the meninges and the surrounding 

 bones, which inclose the cerebro-spinal axis. 



The Neural Tube (Figs. 16 and 114). It is well formed at 

 the fifteenth day. The cephalic end of the tube is much larger 

 than the caudal end, and presents at this time two constrictions 

 that separate the primary brain vesicles from one another 

 the anterior, the middle, and the posterior. Behind the posterior 



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