184 THE FIFTH DAY. [CHAP. 



above would be posterior and below anterior. These latter terms it will be 

 henceforward most convenient to adopt. 



The exact shape/however, varies according to the region 

 of the body from which the section is taken. 



The epiblast walls are at this time composed of radiately 

 arranged columnar cells. The cells are much elongated, but 

 s-omewhat irregular; and it is very difficult in sections to 

 make out their individual boundaries. They contain granu- 

 lar oval nuclei in which a nucleolus can almost always be 

 seen. The walls of the canal are both anteriorly and pos- 

 teriorly considerably thinner 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 some- 

 what bulbous enlargement, producing an hour-glass appear- 

 ance (Fig. 44). Its walls however still preserve the same 

 histological characters as before. 



On the fourth day (Fig. 47) coincidently with the appear- 

 ance of the spinal nerves, important changes may be observed 

 in the hitherto undifferentiated epiblastic walls. 



In the anterior region of the cord, the external portions of 

 the epiblast become modified into grey matter, forming an 

 anterior grey column, which in turn is covered superficially by 

 a mass of white matter forming an anterior white column. 

 The internal portions of the epiblast remain as the epithe- 

 lium lining the spinal canal. Both columns are formed at 

 the point of entrance of the anterior nerve-roots ; and these 

 may easily be traced through the white into the grey matter. 



The grey column is composed of numerous small nuclei, 

 each of which appears to be surrounded by a definite mass of 

 protoplasm, though the boundaries of the protoplasm belong- 

 ing to each nucleus can only occasionally be made out. 

 The nuclei lie in the meshes of a network of fibres continu- 

 ous with the fibres of the nerve-root, and passing through 

 the mass of grey matter in two directions : (1) round the 

 anterior end of the spinal canal, immediately outside its 

 epithelium and so to the grey matter on the opposite side, 

 forming in this way an anterior commissure through which a 

 decussation of the fibres from the opposite sides is effected : 

 (2) upwards along the outside of the lateral walls of the canal. 



