126 



EPIBLAST. 



what pyriform medullary plate, the sides of which form the 

 projecting medullary folds (fig. 77 A). The medullary plate is 

 thickened at the two sides, and is grooved in the median line by 

 a delicate furrow (fig. 72, r). The dilated extremity of the 

 medullary plate, situated at the end of the embryo opposite the 

 blastopore, is the cerebral part of the plate, and the remainder 



FIG. 72. TRANSVERSE SECTION THROUGH THE POSTERIOR CEPHALIC REGION OF 



AN EARLY EMBRYO OF BOMBINATOR. (After Gotte.) 



/. medullary groove; r. axial furrow in the medullary groove; h. nervous layer of 

 epidermis ; as. outer portion of vertebral plate ; is. inner portion of vertebral plate ; 

 s. lateral plate of mesoblast ; g. notochord ; e. hypoblast. 



the spinal. The medullary folds bend upwards, and finally 

 meet above, enclosing a central cerebro-spinal canal (fig. 

 74). The point at which they first meet is nearly at the 

 junction of the brain and spinal cord, and from this point their 

 junction extends backwards and forwards; but the whole 

 process is so rapid that the closure of the medullary canal for its 

 whole length is effected nearly simultaneously. In front the 

 medullary canal ends blindly, but behind it opens freely into the 

 still persisting blastopore, with the lips of which the medullary 

 folds become, as in other types, continuous. Fig. 73 represents 

 a longitudinal section through an embryo, shortly after the 

 closure of the medullary canal (nc) ; the opening of which into 

 the blastopore (x) is clearly seen. 



On the closure of the medullary canal, its walls become 

 separated from the external epiblast, which extends above it as 

 a continuous layer. In the formation of the central nervous 

 system both strata of the epiblast have a share, though the main 

 mass is derived from the nervous layer. After the central 



