19G 



F(E TA L MEMBRA NE$. 



for the inner face of the zona radiata. The space between this mem- 

 brane 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 dis- 

 tinguishable. 



FIG. 147*. DIAGRAM OF THE FCETAL 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; .17. mesoblast of embryo; H. hypoblast of embryo ; UV. 

 umbilical vesicle ; al. allantois; ALC. allautoic cavity. 



While the above changes are taking place in the amnion, the 

 allantois grows out from the hind-gut as a vesicle lined by hypoblast, 

 but covered externally by a layer of splanchnic mesoblast (fig. 147, 3 

 and 4, al) 1 . The allantois soon becomes a flat sack, projecting into the - 

 now largely developed space between the subzonal membrane and the 

 amnion, on the dorsal side of the embryo (fig. 147*, ALC}. In some 

 cases it extends so as to cover the whole inner surface of the subzonal 

 membrane; in other cases again its extension is much more limited. 

 Its lumen may be retained or may become nearly or wholly aborted. 

 A fusion takes place between the subzonal membrane and the adjoin- 

 ing mesoblastic wall of the allantois, and the two together give rise 



1 The hypoblastic element in the allautois is sometimes very much reduced, so that 

 the allantois may be mainly formed of a vascular layer of mesoblast. 



