116 



THE FEOG. 



curves slightly downwards, round the end of the notochord, but 

 ends blindly a long distance from the mesenteron. A string of 

 cells, connecting the two structures, is at this stage the sole indi- 

 cation of the former communication between them. 



The neural tube, formed in the way described above, by 

 fusion of the neural folds, soon separates along its entire length 

 from the external epiblast, and by thickening of its walls and 

 various histological changes becomes converted into the central 

 nervous system ; the anterior part forming the brain, and the 

 posterior part the spinal cord. The lumen or cavity of the 

 neural tube persists throughout life as the central canal of the 



H s N 



1 i 



NC 



FIG. 60. Sagittal section of a Frog Embryo shortly before closure of the 

 blastopore, and of the same age as the embryo shown in Fig. 58, D- x 30. 



B, blastopore. BF, fore-brain. BH, hind-brain. BM, mid-brain, H, hypoblast. 

 L, liver. M, mesoblast. MN, mesenteron. N, notochord. NC, neurenteric canal. 

 P, ingrowth of epiblast to form the pituitary body. PD, proctodfeum. R, rectal 

 diverticulum of mesenteron. S, central canal of spinal cord. Y, yolk cells. 



spinal cord and the ventricles of the brain (cf. Figs. 60, 61, 64, 

 and 65). 



The further changes undergone by the spinal cord are com- 

 paratively slight, and will not be described in detail. Almost 

 from the first (Fig. 70, p. 147), the spinal cord is oval in transverse 

 section, the central canal being a vertical slit. The layer of 

 cells lining the central canal, derived (cf. Fig. 59) from the 

 outer or epidermic layer of the epiblast, remains throughout life 

 as a layer of columnar ciliated epithelial cells ; while the outer 

 wall of the neural tube, formed from the deeper or nervous layer 



