NERVOUS SYSTEM OF THE VERTEBRATA. 415 



THE CENTRAL NERVOUS SYSTEM OF THE VERTEBRATA 1 . 



The formation of the cerebro-spinal axis of the Chordata 

 from the medullary plate has already been treated at length 

 (pp. 301 304). Before entering into the consideration of the 

 morphological value of the various parts of this cord, it will be 

 convenient to describe the more important features of its 

 ontogeny. For this purpose the two parts into which the 

 nervous axis becomes at an early period divided, viz. the spinal 

 cord and the brain, may be dealt with separately. 



The Spinal Cord, shortly after the closure of the medullary 

 canal, has, in all the true Vertebrata, the form of an oval tube ; 

 the walls of which are of a fairly uniform thickness, and are 

 composed of several rows of elongated cells. This cord, as 

 development proceeds, usually becomes vertically prolonged in 

 transverse section, and the central canal which it contains also 

 becomes vertically elongated. The variations in shape of the 

 spinal canal are very great at different periods and in different 

 parts of the body, and an attempt to chronicle them would 

 appear, in the present state of our knowledge, to be quite 

 valueless' 2 . Fig. 117, in which the spinal cord of the chick 

 of the third day is shewn in transverse section, illustrates the 

 character of the cord at the stage just described. Up to this 

 time the walls of the spinal canal have exhibited an uniform 

 structure. A series of changes now however takes place, which 

 results in the differentiation (i) of the epithelium of the central 

 canal, (2) of the grey matter of the cord, and (3) of the external 

 coating of white matter. 



The relative time at which each of these parts becomes 

 developed is not constant in the different forms. 



The white matter is apparently the result of a differentiation 

 of the outermost parts of the superficial cells of the cord into 



1 For the development of the central nervous system in Amphioxus and the 

 Tunicata the reader is referred to the chapters dealing with those two groups. 



2 Lowe (No. 341) holds that at an early stage of development three regions can 

 always be distinguished in any section of the central canal, viz. (i) a ventral narrow 

 slit, (2) a median enlargement, and (3) a dorsal slit. Such a form can no doubt often 

 be observed, but my own observations do not lead me to attach any special importance 

 to it. 



