522 



THE HUMAN EMBRYO. 



ponent epithelial cells remain throughout life in an indifferent 

 state, and give rise to the intrinsic skeletal framework of the 

 adult cord, while other cells become modified to form the nerve 

 cells and nerve fibres ; the nerve fibres arising, at any rate in 



(MX 



Fia. 225. — A transverse section through a portion of the wall of the spinal 

 cord of a Human Embryo at the beginning of the fourth week. The 

 entire thickness of the wall is represented. The upper border of the 

 figure corresponds to the inner surface of the spinal cord, next to the 

 central canal ; the lower border of the figure to the outer surface of the 

 spinal cord. (From His.) x 750. 



ETI, nuclei of the spongioblasts. K^K, processes of the sponjfioblasts which unite 

 to foiin the network or myelospongiiim. N"X, germinal cell. ]S"Z, neuroblast. 



the first instance, as direct prolongations of the protoplasmic 

 bodies of the nerve cells. 



The spinal cord consists at first of a single layer of columnar 

 epithelial cells, each cell extending the whole thickness of the 

 wall. In the mid-dorsal and mid-ventral lines, where the wall 

 is thin, the cells are comparatively short ; but at the sides they 

 are greatly elongated. As commonly happens in columnar epi- 

 thelium, the nuclei of the several cells are placed at different 



