706 



MAMMALIA. 





the embryo, arching over it to form the amnion. Over 

 the embryo the folds of amnion meet in a cupola, and 

 the inner layers of the double fold unite to form the 

 "amnion proper," while the outer layers also unite to 

 form a layer lying internally to the epiblastic blastocyst 

 wall, and termed by Sir William Turner the subzonal 

 membrane. The folds of amnion are continued, as the 

 diagram shows, ventrally as well as dorsally, so that the 



subzonal membrane 

 surrounds the embryo 

 beneath the blasto- 

 cyst wall, while a 

 splanchnic layer of 

 mesoblast grows 

 round about the hy- 

 poblastic yolk - sac. 

 The space between 

 '/ the two layers of 

 .-^ mesoblast, which are 

 termed somatopleure 

 and splanchnopleure, 

 is continuous with the 

 body cavity of the 



FIG. 349. -Diagram of foetal membranes.- embryo. The epi- 



After Turner. blastic outer wall or 



Jj,, Embryo; H., gut lined by hypoblast, dotted; trophoblast, and the 

 the dark is mesoblast; C/.K., umbilical vesicle or , . 3 



yolk-sac; AC., amniotic cavity; am., amnion mCSODlaStlC SUDZOlial 



proper; sz., subzonal membrane ; ALC., allantoic ryipmhrnn 



cavity; /., allantois; zp., may be here taken to 111C1UU1 * 



represent the early epiblastic trophoblast. The eluded 111 

 figure does not show that the amnion folds consist v -\ 



of both epiblast and mesoblast. term- - dlplOtrOphO- 



blast. (5) From the 



hind-wall of the gut there grows out a hypoblastic sac, the 

 allantois, insinuating itself and spreading out in the space 

 between the two layers of mesoblast. As an outgrowth of 

 the gut, homologous with the bladder of the frog, the 

 allantois is lined by hypoblast or endoderm, but it is 

 covered externally by a layer of mesoblast, which it bears 

 with it as it grows. In all placental Mammals, the allantois, 

 which becomes richly vascular, unites with the subzonal 

 membrane, and therefore with the external epiblast as well, 

 to form the foetal part of the placenta, with outgrowing 



n 



