104 



EPJ BLAST. 



stituted throughout of two strata. The formation of the medullary 

 canal commences by the nervous layer along the axial dorsal line 

 becoming thickened, and giving rise to a somewhat pyriform medul- 

 lary 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 the spinal. The medullary fold:^ 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 



Fig. 72. Tkaxsverse section through the posteriok cephalic region op 



AN EARLY EMBRYO OF BOMBINATOR. (After GottC.) 



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

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

 s. lateral plate of mesoblast; g. notochord; <j. hypoblast. 



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 nervous tube has become separated 

 from the external skin, the two layers forming it fuse together ; but 

 there can be but little doubt that at a later period the epidermic 

 layer separates itself again as the central epithelium of the nervous 

 system. 



Both the nervous and epidermic strata have a share in forming the 



