356 THE CAT. [CHAP. x. 



of the mesoblast, open downwards and lined with hypoblast, which 

 gradually becomes converted into an elongated canal, from which 

 secondary, more or less ramifying tubes (also lined with continua- 

 tions of hypoblast) grow out. 



The nervous system also arises as a longitudinal axial groove of 

 the mesoblast (lined with epiblast) extended above the alimentary 

 groove and open in the opposite direction, namely, upwards. This 

 groove also becomes converted into a canal and forms the basis of 

 the cerebro-spinal axis, but the mass of the nerves do not arise as 

 outgrowths from it, but by modification and transformation of parts 

 of the mesoblast into nerve substance, such incipient nerves becoming 

 attached to the independently formed cerebro-spinal axis. 



The medullary groove (except its more anterior part, which 

 becomes the brain, as hereinafter described,) becomes the SPINAL 

 MARROW, as follows : The ascending dorsal lamina) meet together 

 above and convert the groove into a canal, first, in the cervical 

 region. The layer of epiblast which lines the canal thickens very 

 much on each side, its innermost layer (of columnar cells) becoming 

 epithelium, its outer layer becoming the grey matter of the spinal 

 cord, and consisting of numerous small nuclei, each surrounded by an 

 aggregation of protoplasm. Outside this the white matter of the 

 spinal cord is formed by transformation of the cells of the adjacent 

 portion of mesoblast. The central canal of the developing cord 

 having become relatively elongated from above downwards through 

 the lateral thickening of its walls, becomes next constricted by an 

 ingrowth from each such lateral wall. These ingrowths continue till 

 they meet, the canal becoming thus divided by a transverse partition 

 into a dorsal and ventral canal. Then the roof of the dorsal canal 

 becomes absorbed, and this dorsal part of the primitive medullary 

 canal (now again become a groove) is the posterior (dorsal) median 

 fissure of the adult spinal cord. Meantime the white substance of 

 the ventral aspect of the developing myelon grows out on each side, 

 leaving, however, a median interspace which becomes in the adult 

 the anterior (ventral) median fissure, which has thus quite another 

 origin and nature from the posterior (dorsal) median fissure. That 

 part of the primitive canal which is on the ventral side of the trans- 

 verse partition (formed by the coalesced lateral ingrowths above 

 mentioned,) persists as the canalis centralis of the spinal cord of the 

 adult. The cervical and lumbar enlargements of the spinal cord 

 soon make their appearance, and the canalis centralis is also some- 

 what enlarged in those two regions. The myelon is at first 

 co-extensive with the neural canal of the skeleton, so that there is 

 no cauda equina. Afterwards, however, its growth does not keep 

 place with that of the skeleton, and thus the roots of the hindermost 

 spinal nerves become more and more elongated, and the formation 

 of a cauda equina thence results. 



At its proximal end the spinal cord merges into the medulla 

 oblongata by the thickening of the floor and sides of the primitive 



