Shortly after the establishment of the circulation of the yolk-sack the folds of the amnion meet and coalesce above the embryo (fig. 147, 3 and 4, _am_). After this the inner or true amnion becomes severed from the outer or false amnion, though the two sometimes remain connected by a narrow stalk. Between the true and false amnion is a continuation of the body cavity. The true amnion consists of a layer of epiblastic epithelium and generally also of somatic mesoblast, while the false amnion consists, as a rule, of epiblast only; though it is possible that in some cases (the Rabbit?) the mesoblast may be continued along its inner face. [FIG. 147*. DIAGRAM OF THE FOETAL MEMBRANES OF A MAMMAL. (From Turner.) Structures which either are or have been at an earlier period of development continuous with each other are represented by the same character of shading. _pc._ zona with villi; _sz._ subzonal membrane; _E._ epiblast of embryo; _am._ amnion; _AC._ amniotic cavity; _M._ mesoblast of embryo; _H._ hypoblast of embryo; _UV._ umbilical vesicle; _al._ allantois; _ALC._ allantoic cavity.] Before the two limbs of the amnion are completely severed, the epiblast of the umbilical vesicle becomes separated from the mesoblast and hypoblast of the vesicle (fig. 147, 3), and, together with the false amnion (_sh_), with which it is continuous, forms a complete lining for the inner face of the zona radiata. The space between this membrane and the umbilical vesicle with the attached embryo is obviously continuous with the body cavity (_vide_ figs. 147, 4 and 147*). To this membrane Turner has given the appropriate name of subzonal membrane: by Von Baer it was called the serous envelope. It soon fuses with the zona radiata, or at any rate the zona ceases to be distinguishable.