428 



EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



presents three microscopic layers, like those seen earlier in the 

 brain vesicles. 



The outer layer, or marginal velum, composed of neuroglia, 

 is very thin except in the central fissure and in the floor and roof 

 of the tube on either side of the median line. In those situations 

 are formed the lateral, the anterior and the posterior funiculi of 

 the cord. 



The mantle layer is gray matter. It contains the neuro- 

 blasts. At the fourth or sixth week it is very thick, comprising 

 nearly all of the neural tube. The H-shaped column of gray sub- 



Pbsterwr 



Fig. 128. Mode of origin of anterior and posterior roots of spinal nerves. Diagram- 

 matic. (Brubaker and Edinger after His.) 



stance is derived from this layer. In the ventral zone the neuro- 

 blasts develop earliest. They collect near the floor of the tube 

 and form a large column on either side of the median line, the an- 

 terior columna. The axones of the more ventrally located neuro- 

 blasts grow centrifugally out of the antero-lateral surface of the 

 neural tube. They form the anterior roots of the spinal nerves. 

 The more dorsally located neuroblasts develop axones also; but 

 on account of meeting resistance, according to His, they become 

 longitudinal and form the fasciculi proprii of the cord and, per- 

 haps, the ascending anterior cerebello-spinal and the spino-thala- 



