260 



HAND-BOOK OF PHYSIOLOGY. 



This communication between the embryo and the yelk is gradually 

 contracted by the further tucking in of the blastoderm from all sides, 



The head-fold has 

 fold of the 



splanchnopleure; the line of reference, FSo, lies outside the embryo in the "moat, 11 which marks off 

 the overhanging head from the amnion; Z>, inside the embryo, is that part which is to become the 

 fore-gut; FSo and FSp, are both parts of the head-fold, and travel to the left of the figure as develop- 

 ment proceeds; pp, space between somatopleure and splanchnopleure, pleuro-peritoneal cavity; Am. 

 commencing head-fold of amnion; NC, neural canal; Ch, notochord; Ht, heart; A, B, C, epiblast, 

 mesoblast, hypoblast. (Foster and Balfour.) 



till it become narrowed down, as by an invisible constricting band, to a 

 mere pedicle which passes out of the body of the embryo at the point of 

 the future umbilicus. 



FIG. 416. Diagrammatic section showing the relation in a mammal between the primitive alimen 

 tary canal and the membranes of the ovum. The stage represented in this diagram corresponds tc> 

 that of the fifteenth or seventeenth day in the human embryo, previous to the expansion of the al- 

 lantois; c, the villous chorion; a, the amnion; a', the place of convergence of the amnion and reflex- 

 ion of the false amnion a" a", or outer or corneus layer; e, the head and trunk of the embryo, com- 

 prising the primitive vertebrae and cerebro-spinal axis; t, t, the simple alimentary canal in its upper 

 and lower portions. Immediately beneath the right hand i, is seen the foetal heart, lying in the an- 

 terior part of the pleuro-peritoneal cavity; v, the yolk-sac, or umbilical vesicle; v i, the vitello-intes- 

 tinal opening; it, the allantois connected by a pedicle with the anal portion of the alimentary canal. 

 (From Quain n s "Anatomy.") 



Visceral Plates. The downwardly folded portions of blastoderm 

 are termed the visceral plates. 



