THE NERVOUS SYSTEM. 249 



The anterior end of the head of the embryo is lifted up above 

 the blastoderm by the head fold (c/. Fig. 112) ; and the neural 

 folds are continued round this uplifted head to its under surface 

 (Fig. 110), where they become continuous with each other in the 

 median plane. A transverse section across the extreme anterior 

 end of an embryo at this stage (Fig. 110) will cut the projecting 

 neural folds, but no other part of the embryo, and will consist of 

 two completely separate halves. 



By the middle of the second day (Fig. Ill) the neural folds 

 have met and fused, so as to complete the neural tube, along the 

 whole length of the brain region ; the last point to close being 

 in the position afterwards occupied by the pineal body. The 

 fusion has also extended backwards along the greater part of the 

 region of the spinal cord, but at the hinder end of the embryo 

 the two neural folds are still a little distance apart. 



2. The Spinal Cord. 



The spinal cord, in the earlier stages of its development, is 

 oval in transverse section (Fig. 129, NS) : its roof and floor, in the 

 mid-dorsal and mid-ventral planes, remain thin ; but its side walls 

 thicken, so as to reduce the central cavity to a narrow vertical slit. 



In the side walls of the spinal cord a distinction is present, 

 almost from the first, between (i) an inner layer (jf columnar 

 ciliated epithelial cells, lining tJie central canal ; and (ii) the 

 cells composing the rest of the thickness of the wall. These 

 latter apparently do not give rise to either nerve-cells or nerve- 

 fibres, but become modified to form a supporting framework to 

 the cord. The cells of this second group are from the first 

 radially arranged, and during the second day they branch at 

 their outer ends, the branches anastomosing with those of adja- 

 cent cells to form a delicate reticular framework. 



In the meshes of this reticulum certain other cells, the 

 neuroblasts, appear during the third day ; these are apparently 

 derived, by direct modification, from certain of the columnar 

 epithelial cells lining the central canal, which migrate outwards 

 into the reticulum. Each neuroblast is at first bipolar, having 

 a shorter process, directed inwards towards the central canal ; 

 and a longer process which is directed outwards, and which by 

 further growth becomes the axis cylinder of a nerve fibre. The 

 axis cylinders thread their way through the meshes of the 



