THE PLACENTA. 541 



cells enclosing another aggregate of elements — the future 

 inner layer, endoderm or hypoblast. (2) The epiblast 

 divides into an embryonic . disc which will form the 

 epidermis, nervous system, etc., of the embryo, and an ex- 

 ternal layer, the wall of the embryonic sac or blastocyst, with 

 which the disc retains a slight connection until the protective 

 amnion is formed. In the outer epiblastic wall lacunae 

 develop, which are bathed by the maternal blood, and the 

 pillars of tissue between the lacuna grow out into villi, which 

 aid in this earliest connection between mother and offspring. 

 Long before any vascular area or foetal placenta is developed, 

 the outer epiblastic wall has the above nutritive function, 

 and well deserves its name of trophoblast. (3) The hypo- 

 blast or inner mass, which is at first a solid aggregate of 

 cells, becomes a sac, just as a morula may become a 

 blastosphere. The upper part of this sac forms the lining 

 of the incipient gut, while the lower portion, following 

 the contour of the blastocyst wall, becomes the yolk- 

 less 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 meso- 



