THE SPINAL COED. 523 



levels, and so cause the epithelium to appear as though two or 

 more cells thick. 



These columnar epithelial cells are spoken of as spongio- 

 blasts, and give rise to the skeletal framework of the spinal 

 cord. At the beginning of the fourth week (Fig. 225) each 

 spongioblast is greatly elongated, and consists of a central body, 

 which incloses an oval nucleus, Ni, and from which two main 

 processes arise, inner and outer. The inner process, which is 

 directed towards the central canal of the spinal cord, is broad, and 

 usually unbranched ; it reaches the inner surface of the cord, 

 where it expands to form a wide foot, which unites with those of 

 adjacent spongioblasts to form a continuous lining to the central 

 canal, the membrana limitans interna. These inner processes 

 vary in length in different spongioblasts, according to the posi- 

 tion of the nuclei ; they are all striated longitudinally. 



The outer processes of the spongioblasts, though retain- 

 ing a generally radial direction, branch freely : towards their 

 outer ends they form flattened expansions, which unite with one 

 another, and with the processes of adjacent spongioblasts, to 

 form a reticulum, the myelospongium, NK. The outer ends of the 

 branches reach the membrana limitans externa, on the outer 

 surface of the spinal cord. 



The cells forming the mid-dorsal and mid-ventral walls of 

 the spinal cord remain much shorter than those of the sides, but 

 undergo similar changes. 



The germinal cells. Between the inner ends of the spongio- 

 blasts, close to or in contact with the internal limiting mem- 

 brane, large spherical cells (Fig. 225, NX) are formed ; these have 

 large nuclei, and usually show mitotic figures, indicating active 

 cell-division. These germinal cells, as they are called, appear 

 about the beginning of the fourth week ; they are at first few, 

 but rapidly increase in number, and by the end of the week 

 form an almost continuous layer along the inner surface of the 

 spinal cord. The mode of origin of these germinal cells has not 

 been very clearly determined ; but it appears certain that they 

 are derived from the spongioblasts, and probably by direct 

 modification of these. It is also uncertain whether the formation 

 of germinal cells is limited to the inner surface of the spinal cord, 

 or whether it may occur at all parts of its thickness. 



The neuroblasts (Fig. 225, xz) are pear-shaped cells, which 



