THE NERVOUS SYSTEM 



387 



cells, and an outer marginal layer of fibers free from cells. The marginal 

 layer increases in thickness by the addition of fibers which grow lengthwise 

 of the cord. By the addition of myelin sheaths to these fibers, the 

 marginal zone is converted into the white matter of the cord. 



As the lateral walls increase in thickness, the lumen of the cord is 

 narrowed down so that in a cross section it appears as a slit which, how- 

 ever, widens out slightly at the level of the longitudinal sulcus. Finally, 

 the lateral walls fuse together, except in the ventral region, where a 

 portion of the lumen is left as the central canal of the cord. The plane 

 of fusion of the ependymal cells persists in the adult as the dorsal septum. 

 On the ventral side of the cord, however, a median fissure is formed as a 

 result of the increase in thickness of 

 the lateral walls and the failure of 

 the floor plate to grow. 



The mantle layer becomes the 

 gray matter of the cord. As a 

 result of unequal growth of this 

 layer, dorsal, lateral and ventral 

 gray columns develop. By the 

 time the human embryo is three 

 months old, the gray matter has 

 assumed in cross section its charac- 

 teristic H-shape. 



In the early stages of the spinal 

 cord, two kinds of cells are differ- 

 entiated, germinal cells which be- 

 come neuroblasts, and non-nervous 



spongioblast cells. Following the the bundle of neurites passes a neuroblast, 



a stimulofugal process is formed. Later by 

 a reverse or stimulopetal process the 

 neuroblast is drawn towards the activating 

 bundle and dendritic processes grow towards 

 the bundle. Successive stages in the proc- 

 ess are shown beginning at the top of the 

 diagram. (Redrawn after Ariens-Kappers.) 



Fig. 342. — A diagram illustrating the 

 theory of neurobiotaxis of Ariens-Kappers. 

 The diagram represents a series of motor 

 neuroblast cells, lying in the wall of the 

 spinal cord, as activt^ted to form neurites 

 by a longitudinal bundle of neurites. As 



outgrowth of the neurite from a 

 neuroblast, a number of dendrites 

 grow in the opposite direction. 

 According to the theory of "neuro- 

 biotaxis" of Kappers, the neurite is 

 stimulofugal, that is, grows away from the source of stimulus, and the 

 dendrites are stimulopetal, that is, they grow towards the stimulus. 

 Nerve cells of two kinds arise, motor, which are Hmited to the anterior 

 and lateral gray columns, and association cells, which may lie in the 

 dorsal column. 



Two types of supporting cells develop from the spongioblasts, ependy- 

 mal cells with elongated processes which extend radially from the central 

 canal to the periphery of the cord, and neuroglia cells which have shorter 

 and more numerous processes and which do not extend through the 

 entire thickness of the wall. Some of the spongioblasts form transient 



