EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



of the indifferent cells all forms of karyokinetic figures may be 

 seen. This fact shows that they are rapidly multiplying. 



In a somewhat later stage of development the indifferent cells 

 wander outward toward the marginal velum and arrange them- 

 selves into a definite layer, known as the mantle layer (Figs. 116 

 and 127). Three concentric zones are clearly visible at this time 

 in a cross section of the neural tube ; the marginal velum, the man- 

 tle layer, and the ependymal layer formed by the bases of the epen- 

 dymal cells. As the indifferent cells multiply in the mantle layer 

 they become specialized into two types, the neuroblasts and the 



Fig. 116. Diagram showing development of neurones in the spinal cord. 

 (McMurrich after Schafer.) 



The c 



(McMurrich after Schd/er.) 



:ircles, indifferent cells; circles with dots, neuroglia cells; shaded cells, germinal cells; 

 circles with cross, germinal cells in mitosis; black cells, nerve-cells. 



spongioUasts. The neuroblasts are the larger. They are the 

 embryonic neurones. Each neuroblast throws out a protoplasmic 

 process which develops into an axone and grows outward into the 

 marginal velum. Within the marginal velum the axone either 

 runs some distance and returns to the mantle layer, or it passes 

 outward through the marginal velum and becomes an efferent or 

 motor root-fiber. Collaterals are given off from the axones, the 

 end-tufts are formed and the medullary sheath is laid down as 

 development proceeds and function begins. While these events 



