250 THE CHICK. 



reticulum and reach the surface of the spinal cord, where some 

 leave it to form the roots of the spinal nerves, while others run 

 longitudinally along its outer surface to form the layer of white 

 matter of the spinal cord. 



From the third, or fourth, to about the tenth day, this 

 process of development of neuroblasts and of nerve fibres pro- 

 ceeds rapidly. The neuroblasts become the nerve cells of the 

 spinal cord, the first cells to. be established being those of the 

 ventral cornua : their inner processes disappear, and from the 

 bodies of the cells fine branching protoplasmic outgrowths arise 

 at a later stage, which anastomose with those of neighbouring 

 cells. As the nerve fibres increase in number, the layer of white 

 matter on the surface of the spinal cord necessarily gains in thick- 

 ness, and the spinal cord rapidly approaches the shape charac- 

 teristic of it in the adult. 



The central cavity of the spinal cord is at first a narrow 

 vertical cleft (Fig. 129). The side walls of the dorsal half of 

 this cleft come in contact with each other and fuse, so as to 

 obliterate the cavity; the ventral half of the cleft persists 

 throughout life as the central canal of the spinal cord. 



Of the two longitudinal fissures of the adult spinal cord, the 

 ventral fissure is a median groove left between the ventral 

 columns of white matter, as these increase in thickness ; it may 

 be recognised on the sixth or seventh day, and by the tenth day 

 is a conspicuous feature in transverse sections of the spinal cord. 



The dorsal fissure is formed in quite different fashion. The 

 white matter of the dorsal surface grows down into the spinal 

 cord, about the ninth day, as a pair of vertical plates ; these are 

 at first separated by a thin median lamina of grey matter ; and 

 it is by absorption of this median lamina that the dorsal fissure 

 is formed. The absorption is a gradual one, and for some time 

 the fissure remains bridged across by slender fibres, derived from 

 the grey matter. 



The neurenteric passages. In the floor of the neural canal, 

 at the hinder end of the body, two or three pit-like depressions 

 appear in the early stages of development, which, although they 

 are usually incomplete, and only rarely open into the mesenteron, 

 still appear to be homologous with the neurenteric passage in. 

 Amphioxus or in the frog. 



