544 



Structure of vERtEBRATA 



food canal beneath, so that a 3 -shaped tube results. The 

 connection between them is called the neurenteric canal 

 (Fig. 307, NE.C), and though it is only temporary, its 

 frequent occurrence is of much interest. 



The wall of the medullary canal becomes very much 

 thickened, the roof and floor grow less rapidly, and thus 

 the cord is marked by ventral and dorsal longitudinal 

 furrows. At the same time, the canal itself is constricted, 

 and persists in the fully formed structure only as a minute 

 canal lined by ciliated epithelium, and continuous with the 

 cavity of the brain. 



In the cord it is usually easy to distinguish an external region of 

 white matter, composed of meduUated nerve-fibres, and an internal 



Fig. 307. — Diagram illustrating the development of the 

 nervous system in an Elasmobranch fish. 



NP., Neuropore, the entrance to the cerebral vesicle (C.V.) ; 

 C.V^., posterior vesicle of the brain ; SP.C, spinal cord ; 

 NC, notochord ; NE.C, neurenteric canal, a communication 

 between the neural canal above and the gut or enteron {ENT.) 

 below, y. indicates the position of the yolk, on the top of 

 which the embryo is lying. 



region of grey matter, containing ganglionic cells and non-medullated 

 fibres. 



• The arrangement of the grey matter, together with the longitudinal 

 fissures, gives the cord a distinct bilateral symmetry, which is sometimes 

 obvious at a very early stage. 



The brain substance is also composed of grey and white matter, 

 but there, at any rate in higher forms, the arrangement is very 

 complicated. 



Cranial nerves.— The origin and distribution of the 

 cranial nerves may be summarised as in the table at top 

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