44 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



is covered by a continuous sheet of epidermis. This process of 

 infolding and fusion proceeds from in front backwards, but at 

 the very front end a small opening (anterior neuropore) may 

 persist for some time. The important points to be kept in 

 mind in this connection are that the central nervous system 

 develops by the infolding l of a part of the primitively external 

 surface of the body, and that the inner surface of the neural 

 tube is morphologically external in origin. 



Before the infolding of the neural tube is completed, its 

 anterior end enlarges, the first step in the process of differen- 

 tiation into brain and spinal cord. The latter division must be 

 described first, since it presents much simpler conditions than 

 does the brain. 



The spinal cord frequently retains, to a certain extent, its 

 tubular character throughout life, although the central canal 

 does not materially increase its primitive diameter. In the 

 earlier stages the cord is oval in section, its sides being thick- 

 ened ; while in the median line, above and below, it is much 

 thinner (Fig. 48). These halves rapidly increase in size while 

 the central median portion lags behind, the result being that 

 the cord soon becomes marked along its ventral surface by a 

 longitudinal groove. Later a corresponding cleft appears on 

 the dorsal surface. These are the anterior and posterior fissures 

 of human anatomy. 



In sections of the adult cord one clearly distinguishes an 

 outer white matter and an inner gray substance ; the latter 

 takes the shape of the letter H, the ends of the uprights being 

 called the horns or cornua, 2 while the cross-bar is produced by 

 fibres running from one half to the other between the bottoms 

 of the fissures and the central canal. The horns extend 

 towards the surface, above and below, thus dividing the white 

 matter of each half of the cord into three columns, dorsal, lat- 

 eral, and ventral ; the lateral column being between the two 

 horns, the dorsal and ventral between the horns and the 



1 In some forms (e.g., teleosts, marsipobranchs, some ganoids) the development of the 

 central nervous system varies considerably from that outlined above, but the final result is 

 the same. 



2 A lateral horn (Fig. 46, cornu lat.) must also be recognized on the grounds of physi- 

 ology and nerve origin. 



