THE CENTRAL NERVOUS SYSTEM. 



115 



neural tube. Very shortly, however, the fusion, by which the 

 neural tube is closed, extends further back, so as to involve the 

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

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

 cation between the neural tube and the mesenteron still persists, 

 however, as a narrow tubular passage, the neurenteric canal 

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



CH 



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, coelom or body cavity. CH, notochord. EE, outer or epidermic layer of epiblast. 

 EN", inner or nervous layer of epiblast. M, niesoblast. ME, outer or somatopleuric 

 layer of niesoblast. MH, inner or splanchnopleuric layer of mesoblast. M"C. neural 

 groove. ND, dorsal root of a spinal nerve. M"S, spinal cord. T, rnesenterou. W, liver 

 diverticulum. Y, yolk. 



The neurenteric canal persists only for a very short time. 

 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 



i 2 



