730 



MAMMALIA. 



of this sac forms the lining of the incipient gut,' while the 

 lower portion, following the contour of the blastocyst wall, 

 forms the lining of the umbilical vesicle (cf. the Chick). 

 From this vesicle or yolk-sac the embryo becomes folded 

 off, and the connection between the two is narrowed, just 

 as in the chick, into a canal the vitelline duct, which is 

 part of the "umbilical cord," entering the embryo at the 

 future navel. (4) Between the epiblast and the hypoblast 



of the embryo, the 

 mesoblast develops, 

 splitting into an 

 outer, parietal, or 

 somatic, and an 

 inner, visceral, or 

 splanchnic layer. 

 The cavity between 

 these is the incipient 

 body cavity. A 

 double fold of so- 

 matic mesoblast, 

 carrying with it a 

 single sheet of epi- 

 blast, rises up round 

 FIG. 400. Diagram of foetal memUanes about the embryo, 

 After Turner. 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 

 blastocyst wall, while a splanchnic layer of mesoblast grows 

 round about the hypoblastic yolk-sac. The space between 

 the two layers of mesoblast is continuous with the body 

 cavity of the embryo. The epiblastic outer wall or tropho- 



E., Embryo ; //., 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 ; al., allantois ; z/., may be here taken to 

 arly epiblas 



., a 



the early epi 

 figure does not show that the amnion folds consist 



represent the early epiblastic trophoblast. Th 



t the 

 of both epiblast and mesoblast. 



