DEVELOPMENT. 



807 



strands for each nerve. They appear later than the posterior roots. 

 The rudiment of the posterior root is differentiated into a proximal 

 round nerve connected to the cord, a ganglion ic portion and a distal 

 portion. To the last the anterior nerve-root becomes attached. 



The Spinal Cord. The spinal cord consists at first of the undiffer- 

 entiated epiblast of the walls of the neural canal, the cavity of which is 

 large, with almost parallel sides. The walls are at first composed of 

 elongated irregular nucleated columnar cells, arranged in a radiate 

 manner. The cavity then becomes narrow in the middle and of an 

 hour-glass shape (fig. 501). When the spinal nerves make their first 



Fig. 501. Diagram of development of spinal cord, c c, central canal; a/, anterior fissure ; p/, 

 posterior fissure ; gr, gray matter; tc, white matter. For further explanation, see text. 



appearance, about the fourth day in the chick, the epiblastic walls be- 

 come differentiated into three parts : (a) the epithelium lining the central 

 canal; (b) the gray matter; (c) the external white matter. The last is 

 derived from the outermost part of the epiblastic walls by the conversion 

 of the cells into longitudinal nerve-fibres. The fibres being without any 

 myelin sheath, are for a time gray in appearance. The white matter 

 corresponds in position to the anterior and posterior nerve-roots, and 

 are the anterior and posterior white columns. It is at first a very thin 

 layer, but increases in thickness until it covers the whole cord. The 

 gray matter too arises from the cells by their being prolonged into fibres. 

 The change in the central cells is sufficiently obvious. The anterior and 

 posterior cornua of gray matter and the anterior gray commissure then 

 appear. The anterior fissure is formed on the fifth day by the growth 

 downward of the anterior cornua of gray matter toward the middle 

 line. The posterior fissure is formed later. The whole cord now be- 

 comes circular. The posterior gray commissure is then formed. 



When it first appears, the spinal cord occupies the whole length of 

 the medullary canal, but as development proceeds, the spinal column 

 grows more rapidly than the contained cord, so that the latter appears 

 as if drawn up till, at birth, it is opposite the third lumbar vertebra, 

 and in the adult opposite the first lumbar. In the same way the in- 

 creasing obliquity of the spinal nerves in the neural canal, as we approach 

 the lumbar region, and the cauda equina at the lower end of the cord, 

 are accounted for. 



Brain. We have seen that the front portion of the medullary canal 



