FCETAL MEMBRANES. 



embryo becomes surrounded by a series of folds of thin somato- 

 pleure, which form a continuous wall all round it. All are 

 drawn gradually over the body of the embryo, and at last meet 

 and completely coalesce (fig. 121, H, I, and 117, Am), all traces 

 of their junction being removed. Beneath these united folds 

 there is therefore a cavity, within which the embryo lies (fig. 121 

 H, ae). This cavity is the cavity of the amnion. 



Each fold is necessarily formed of two limbs, both limbs 

 consisting of epiblast and a very thin layer of mesoblast ; but in 

 one limb the epiblast looks towards the embryo, while in the 

 other it looks away from it. The space between the two limbs 

 of the fold, as can easily be seen in fig. 121, is really part of the 

 space between the somatopleure and splanchnopleure ; it is 

 therefore continuous with the general space, part of which 

 afterwards becomes the pleuroperitoneal cavity of the body, 

 shaded with dots in the figure and marked (//) ; so that it is 

 possible to pass from the cavity between the two limbs of the 

 amniotic folds into the cavity which surrounds the alimentary 

 canal. When the several folds meet and coalesce together above 

 the embryo, they unite in such a way that all their inner limbs 

 unite to form a continuous inner membrane or sack, and all 



JVC. 



f.So 



FIG. 112. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE AXIS OF AN 



EMBRYO. 



The section is supposed to be made at a time when the head-fold has commenced 

 but the tail-fold has not yet appeared. 



F.So. fold of the somatopleure. F.Sp. fold of the splanchnopleure; D. fore-gut. 



//. pleuroperitoneal cavity between somatopleure and splanchnopleure ; Am. com- 

 mencing (head) fold of the amnion. For remaining reference letters vide p. 167. 



their outer limbs a similarly continuous outer membrane or sack. 

 The inner membrane thus built up forms a completely closed 

 sack round the body of the embryo, and is called the amniotic 





