THE CENTBAL NEEVOUS SYSTEM. 



115 



neural tube; V ery shortly, however, the f usion, by which the 

 ne ural tube is gbsed, extends farther back, so as to involvo the 

 lips of the blastopore, and the external opening of the blastopore 

 becomes finally closed (Fig. 58, E, and Fig. 61). The comman i- 

 cation between t he neural tube and the mesentero n still pe rsists, 

 however, as a narrow tubular passage, the neurenteric canal 

 (Fig. 61), passing round the hinder end of the notochord. 



Fig. 59. — Transverse section through a Frog Embryo, at a stage corresponding 

 to Fig. 58, C, and showing the neural folds shortly before they meet each 

 other to complete the neural tube. 



C, ccElom or body carity. CH. notochord. EE, outer or epidermic layer of epiblast. 

 EK", inner or nervous layer of epiblast. M, mesoblast. ME, outer or somatopleuric 

 layer of mesoblast, M^. inner or splancbnopleuric layer of mesoblast. JT C, n eural 

 groove, WD, dorsal root of a spinal nerve. NS, spinal cord. T, mesenteron. "W", liver 

 diverticulum. Y, yolk. 



The neurenteric ca nal persists only for a very short timR 

 In the immediately succeeding stages the tail begins to- 

 lengthen rapidly, carrying the hinder end of the neural tube far 

 away from the mesenteron, and the channel of communication 

 between the two becomes speedily obliterated. At the time of 

 hatching (Fig. 69, p. 146), the hinder end of the neural tube 



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