524 THE HUMAN EMBRYO. 



appear in the earlier part of the fourth week ; they lie at first 

 close to the inner wall of the spinal cord, and are believed to 

 be formed by division of the germinal cells, though it is pos- 

 sible that they may also arise directly from the spongioblasts. 

 Each neuroblast consists of a large ovoid nucleus, surrounded by 

 a thin layer of protoplasm which is produced at one pole into a 

 long, striated tail. The ueuroblasts become the nerve cells of 

 the adult spinal cord, while their tails, by further elongation, 

 become the axis cylinders of the nerves, round which at a later 

 stage the medullary and Schwann's sheaths are formed. 



Each neuroblast at first gives rise to only one process or 

 tail, which is directed towards the outer surface of the spinal 

 cord. After their first formation, the neuroblasts wander out- 

 wards, apparently by their own activity, to the outer layers of 

 the cord, where they lie about the junction of the nuclear and 

 reticular layers of the myelospongium. The bodies of the 

 neuroblasts remain embedded in the spinal cord, but the tails, 

 or axis-cylinder processes, grow outwards, threading their way 

 through the meshes of the myelospongium, and ultimately 

 reaching the outer surface of the spinal cord. 



The neuroblasts increase rapidly in numbers during the 

 fourth week ; they move outwards towards the surface of the 

 cord, and at the end of the week (Fig. 226) form a well-marked 

 layer, NZ, spoken of as the mantle layer, just beyond the 

 nuclei of the spongioblasts, Ni. After the withdrawal of the 

 neuroblasts from the inner surface of the spinal cord, the 

 spongioblasts in this region close in, and become arranged as a 

 layer of columnar cells, which acquire cilia at their free ends, and 

 form the characteristic epithelial lining of the central canal of 

 the spinal cord. 



The motor roots of the spinal nerves. In the latter part of 

 the fourth week, the neuroblasts (Fig. 226) are much more 

 abundant in the ventro- lateral regions of the spinal cord than 

 elsewhere. They soon become arranged more or less definitely 

 in groups, and the axis-cylinder processes, converging to form 

 bundles, grow out beyond the outer surface of the spinal cord 

 and form the ventral or motor roots of the spinal nerves, NV. 

 The first trace of these motor roots appears about the twenty- 

 fourth day, and by the end of the fourth week they are well 

 established along the greater part of the length of the cord. 



