NERVOUS SYSTEM OF THE VERTEBUATA. 345 



The walls of the canal coalesce dorsally, and the coalescence 

 gradually extends ventral wards, so as finally to reduce the central 

 canal to a minute tube, formed of the ventral part of the original 

 canal. The epithelial wall formed by the coalesced walls on the 

 dorsal side of the canal is gradually absorbed. 



The epithelium of the central canal, at the period when its 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 pro- 

 longed so as to line its walls ; and shortly after its formation the 

 dorsal grey commissure makes its appearance, which is not impro- 

 bably derived from part of the epithelium of the original central 

 canal. 



Development of the Br'tin. 



The brain is formed from the anterior portion of the medullary 

 plate. When the medullary plate first becomes differentiated 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 eaily in- 

 dicated by a widening of the medullary plate, but does not become 

 sharply marked off from the region of the spinal cord. In many 

 Ichthyopsida (Elasmobranchii (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 developmental 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 constric- 

 tion, and subsequently the posterior of these again becomes divided 

 into two, so that three lobes are formed known as the fore- the 

 mid- and the hind-brain ; of these the hind-brain is usually the 

 longest. In some instances a bilobed stage can hardly be recognised. 

 This primitive division of the brain is shewn in many of the figures 

 already given. The reader may perhaps best refer to fig. 108. On the 

 closure of the medullary groove the lumen of the medullary canal is 

 continued uninterruptedly through the brain, but dilates considerably 

 in each of the cerebral vesicles. 



