190 



F(ETAL MEMBRANES. 



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 >vith the body cavity {vide figs. 147, 4 and 147*). To 

 this membrane Turner has given the appropriate name of sub zona l 

 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 membkanes 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; VV. 

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



While tVie 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, aiy. 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 



^ The hypoblastic element in the allantois is sometimes very much reduced, so that 

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



