NERVOUS SYSTEM OF THE VERTEBRATA. 419 



atrophy commences, is not covered dorsally either by grey 

 or white matter, so that, with the gradual reduction of the 

 dorsal part of the canal, and the absorption of the epithelial wall 

 formed by the fusion of its two sides, a fissure between the two 

 halves of the spinal cord becomes formed. This fissure is the 

 posterior or dorsal fissure. In the process of its formation the 

 white matter of the dorsal horns becomes prolonged so as to 

 line its walls ; and shortly after its formation the dorsal grey 

 commissure makes its appearance, which is not improbably 

 derived from part of the epithelium of the original central 

 canal. 



Development of the Brain. 



The brain is formed from the anterior portion of the me- 

 dullary plate. When the medullary plate first becomes diffe- 

 rentiated it is not possible to distinguish between the region of 

 the brain and that of the spinal cord. The brain region is 

 however usually very early indicated by a widening of the 

 medullary plate, but does not become sharply marked off from 

 the region of the spinal cord. In many Ichthyopsida (Elasmo- 

 branchii (fig. 28, C) and Amphibia (fig. 77, A)) the anterior 

 dilatation gives to the medullary plate, before its sides meet to 

 form a canal, a spatula-like form ; which is either not present or 

 less marked in Reptilia, Aves and Mammalia. 



The length of the brain as compared to the spinal cord is 

 always very great in the embryo, and in the earliest develop- 

 mental periods the disproportion in the size of the brain is 

 specially marked, owing to the full number of the somites of 

 the trunk not having been formed. In Elasmobranchii the 

 brain is about one-third of the whole length of the embryo at 

 the stage immediately following the closure of the medullary 

 canal. 



The first differentiation of the brain into distinct parts is 

 a very early occurrence, and may take place before (Mammalia) 

 or during the closure of the medullary folds. The brain first 

 becomes divided into two successive lobes or vesicles by a 

 single transverse constriction, and subsequently the posterior of 

 these again 'becomes divided into two, so that three lobes 



272 



