CONNECTION BETWEEN EMBRYO AND MOTHER. 671 



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 - — --'^.J-P 



beneath the blasto- 

 . cyst wall, while a 

 splanchnic layer of 

 mesoblast grows 

 round about the hy- 

 poblasts yolk - sac. 

 The space between 

 the two layers of 

 mesoblast, which are 

 termed somatopleure 

 and splanchnopleure, 

 is continuous with the 

 body cavity of the 

 embryo. The epi- 

 blastic outer wall or 

 trophoblast, and the 

 mesoblastic subzonal 

 membrane, are in- 

 cluded in Hubrecht's 

 term — diplotropho- 

 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 fcetal part of the placenta, with outgrowing 



Fig. 295. — Diagram of foetal membranes. — 

 After Turner. 



E., Embiyo ; H., gut lined by hypoblast, dotted ; 

 the dark is mesoblast ; UV., umbilical vesicle or 

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

 proper ; sz. } subzonal membrane ; ALC, allantoic 

 cavity; a/. } allantois; z/>., may be here taken to 

 represent the early epiblastic trophoblast. The 

 figure does not show that the amnion folds consist 

 of both epiblast and mesoblast. 



