252 THE FIFTH DAY. [CHAP. 



difficult in sections to make out their individual 

 boundaries. They contain granular oval nuclei in 

 which a nucleolus can almost always be seen. The 

 walls of the canal are both anteriorly and posteriorly 

 considerably thinner in the median plane than in the 

 middle. 



Towards the end of the third day changes take 

 place in the shape of the cavity. In the lumbar region 

 its vertical section becomes more elongated, and at the 

 same time very narrow in the middle while expanded 

 at each end into a somewhat bulbous enlargement, pro- 

 ducing an hour-glass appearance (Fig. 65). Its walls 

 however still preserve the same histological characters 

 as before. 



On the fourth day (Fig. 68) coincidently with the 

 appearance of the spinal nerves, important changes 

 may be observed in the hitherto undifferentiated epi- 

 blastic walls, which result in its differentiation into (1) 

 the epithelium of the central canal, (2) the grey matter 

 of the cord, and (3) the external coating of white 

 matter. 



The white matter is apparently the result of a 

 differentiation of the outermost parts of the superficial 

 cells of the cord into longitudinal nerve-fibres, which 

 remain for a long period without a medullar}^ sheath. 

 These fibres appear in transverse sections as small dots. 

 The white matter forms a transparent investment of 

 the grey matter; it arises as four patches, viz. an anterior 

 and a posterior white column on each side, which lie on 

 a level with the origin of the anterior and posterior 

 nerve-roots. It is always, at first, a layer of extreme 

 tenuity, but rapidly increases in thickness in the sub- 



