MEMBRANES OF THE EMBRYO- AMNION. 



659 



Membranes of the -Embryo. Amnion. The mesoblast 

 about the embryo splits into two laminae, the parietal and the 

 visceral. The parietal (external) joins with the epiblast to form 

 the somatopleure, from which the amnion and the body-walls are 

 developed, while the visceral lamina unites with the hypoblast to 



F.So 



FIG. 441. Diagrammatic longitudinal section through the axis of an embryo 

 chick : N, C., neural canal ; Ch, notochord ; D, foregut ; F. So, somatopleure ; F. Sp, 

 splanchnopleure ; Sp, splanchnopleure forming the lower wall of the foregut ; Ht, 

 heart ; pp, pleuroperitoneal cavity ; Am, amniotic fold ; A, epiblast ; B, mesoblast ; 

 C, hypoblast (Foster and Balfour). 



form the splanchnopleure. From this structure are developed the 

 walls of the allantois, the yolk-sac, and the alimentary canal. 

 Between the somatopleure and the splanchnopleure is the pleuro- 

 peritoneal cavity, which later is divided by partitions into peri- 

 cardial, pleural, and peritoneal cavities. From the somatopleure 

 folds form which rise above the embryo on all sides, meeting over 



hy 



FIG. 442. Diagrammatic longitudinal section of a chick on the fourth day : ep, 

 epiblast ; liy, hypoblast ; sm, somatopleure ; win, splanchnopleure ; af, pf, folds of the 

 amnion ; pp, pleuroperitoneal cavity ; am, cavity of the amnion ; ai, allantois ; a, 

 position of the future anus; h, heart; i, intestine; m, vitelline duct; ys, yolk; s, 

 foregut ; m, position of the mouth ; me, the mesentery (Allen Thomson). 



its back and fusing together. These are the amniotic folds. As 

 each fold is double, when they unite two membranes result : the 

 inner, next the embryo, is the amnion, and the outer, toward the 

 vitelline membrane, is the false amnion (Fig. 442). The latter 

 and the vitelline membrane fuse together, forming the chorion. 



