90 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



vesicle has not yet reached the uterus, that a grooved strip of 

 epiblast, the medullary plate, in front of the primitive streak 

 (Fig. 68) is set aside, by a process of infolding, to form the great 

 central nervous system — the brain, spinal cord, and nerves 

 (Fig. 69). 



A medullary fold rises up on each side of what is to be the 

 middle line of the back to enclose the medullary plate. The 

 folds rise until their crests meet, and a tube of epiblast is buried 

 by the fusion of their lips. The blastopore is included within 

 the posterior end of the medullary folds (Fig. 68). Thus, for a 

 short space, the cavity of the yolk sac communicates with the 

 neural canal. 



The Notochord. — Beneath the neural tube a similar infolding 

 of hypoblast takes place, and a tube of cells to form the noto- 



notochord^ 

 hypoblast^ 



neural canal 



somatopleure. 

 splanchno-pleure 



'piblast 



mesoblast 



fold of amnion 



r intermediate 

 ^{cell-mass. 



outer layer of mesoblast 



inner layer of 

 mesoblast 



coelom 



Flo. 69. — Diagrammatic section of a Blastodermic Vesicle showing (1) the origin of 

 the neural canal, (2) the origin of the notochord, (3) the" ingrowth of the 

 mesoblast, and (4) the formation of the coelom. 



chord is detached (Fig. 69). It forms the first basis of the 

 spinal column. 



The Somatopleure and Splanchnopleure. — The mesoblast 

 surrounds both the notochord and neural canal as they are- 

 formed — perhaps assists in their formation (Figs. 69 and 70). 

 The solid layer which surrounds these structures is known as the 

 paraxial mesoblast. As the mesoblastic cells spread out to cover 

 the outer aspect of the hypoblast and inner aspect of the 

 epiblast, they separate into two layers which enclose between 



