350 The Development of the IS'euroglia 



but more slowly and always by a collapse of its dorsal portion, till at 70 

 millimeters a central canal results which is but slightly larger than the 

 adult. 



The mantle layer from its first appearance completely encompasses 

 the ventral aspect of the specimen, while on the dorsal aspect the ependy- 

 mal layer for a time extends to the very periphery {d. Fig. 6). With 

 the collapse of the ventricle and the further lateral growth of the speci- 

 men, the mantle layer closes about the dorsal aspect also. Then, as the 

 lateral growth of the embryonic spinal cord continues, depressions nat- 

 urally result at both ends, of the mid-line. Subsequently, the growth 

 goes on in such a way that, as the dejjressions deepen, the one on the ven- 

 tral aspect remains open as the anterior median fissure, while the dorsal 

 one collapses almost as fast as it is formed, and becomes the posterior 

 septum. What for a time is apparently a portion of the posterior septum 

 extending through the nucleated layers, is only the remains of the inter- 

 nal limiting membrane {ms. Fig. 8). As the neuraxes grow in, this 

 appearance is obliterated by the dorsal commissure, etc. The investing 

 pial tissue accompanies the depressions and aids in maintaining the pos- 

 terior septum of the adult. 



ISTo nerve axones are discernible in my sections of the spinal cord of 

 pigs at 15 millimeters. At 35 millimeters neuraxes have begun to ap- 

 pear in transverse sections as fine dots embedded in the syncytium and 

 staining like it. They are more evident in the mantle layer, especially 

 in the dorsal portion. At 30 millimeters (Figs. 7 and 8) the syncytium 

 of the mantle layer is more thickly studded with axones, and from 30 

 millimeters upward they become more abundant and more generally dis- 

 tributed and show a slight and gradual increase in size. Until the pro- 

 cesses of medullation begin (16 to 30 centimeters), the ordinary methods 

 show them simply as dots and staining only a darker shade of the same 

 color as the syncytium which closely invests them. The silver method, 

 of course, differentiates them clearly. 



In the mantle layer the radial arrangement of the syncytium is prac- 

 tically perpendicular to the periphery except at the ventral aspect of the 

 ventricle {mv. Figs. 6 to 9). Here the more rapid lateral direction in 

 the growth of the wall of the neural tube results in the syncytium being 

 drawn into an arrangement parallel to the periphery. However, as tlie 

 lateral growth continues and the depression which results in the median 

 fissure appears, the lateral tension decreases and becomes equalized and 

 soon filaments may be seen arranged in both directions {mv, Figs. 8 and 

 9). A suggestion of this result can also be seen through the ventral 

 portion of the middle nucleated layer {mn. Fig. 8). 



