DEVELOPMENT. 697 



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 corre. 

 sponds 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 mat- 

 ter too arises from the cells by their being prolonged into fibres. The 

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

 terior cornua of gray matter and the anterior gray commissure then ap- 

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

 downwards of the anterior cornua of gray matter towards 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 



FIG. 482. Diagram of development of spinal cord; cc, central canal; a/, anterior fissure; pf, 

 posterior fissure; g, gray matter; w, white matter. For further explanation see text. 



the adult opposite the first lumbar. In the same way the increasing obli- 

 quity of the spinal nerves in the neural canal, as we approach the lum- 

 bar 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 

 is almost from the first widened out and divided into three vesicles. 

 From the anterior vesicle (thalamencephalon) the two primary optic 

 vesicles are budded off laterally: their further history will be traced in 

 the next section. Somewhat later, from the same vesicle the rudiments 

 of the hemispheres appear in the form of two outgrowths at a higher 

 level, which grow upwards and backwards. These form the prosenceph- 

 alon. 



In the walls of the posterior (third) cerebral vesicle, a thickening ap- 

 pears (rudimentary cerebellum) which becomes separated from the rest 

 of the vesicle by a deep inflection. 



At this time there are two chief curvatures of the brain (Fig. 483, 3). 

 (1.) A sharp bend of the whole cerebral mass downwards round the 

 end of the notochord, by which the anterior vesicle, which was the high- 



