DEVELOPMENT AND PLACENTATION. 651 



forms the lining of the incipient gut, while the lower por- 

 tion, following the contour of the blastocyst wall, becomes 

 the yolkless yolk sac or umbilical vesicle. Its connection with 

 the upper part is narrowed 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 hypo- 

 blast 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 somatic mesoblast, carrying 

 with it a single sheet of epiblast, rises up round about 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 inter- 

 nally 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, which 

 are shortly termed somatopleure and splanchnopleure, is 

 obviously continuous with the body cavity of the embryo. 

 The epiblastic outer wall or trophoblast, and the mesoblastic 

 subzonal membrane, are included in Hubrecht's term 

 diplotrophoblast. (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 of course 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 vascular processes or villi 

 which fit into corresponding depressions or crypts on the wall 

 of the uterus. [To the mesoblastic wall of the allantois, plus 

 the subzonal membrane, the term " chorion " is sometimes 

 applied, but as the word has been used in many different 



