MAMMALIA. 237 



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 ofsubzonal 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. 



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

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

 blast, but covered externally by a layer of splanchnic mesoblast 

 (fig. 147, 3 and -i, a/) 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 adjoining mesoblastic wall of the allantois, and the two 

 together give rise to a secondary membrane round the ovum, 

 known as the chorion. Since however the allantois does not 

 always come in contact with the whole inner surface of the sub- 

 zonal membrane, the term chorion is apt to be somewhat vague ; 

 and in the rabbit, for instance, a considerable part of the 

 so-called chorion is formed by a fusion of the wall of the yolk- 

 sack with the subzonal membrane (fig. 148). The placental 

 region of the chorion may in such cases be distinguished as the 

 true chorion, from the remaining part which will be called the 

 false chorion. 



The mesoblast of the allantois, especially that part of it 

 which assists in forming the chorion, becomes highly vascular ; 

 the blood being brought to it by two allantoic arteries continued 

 from the terminal bifurcation of the dorsal aorta, and returned 

 to the body by one, or rarely two, allantoic veins, which join the 

 vitelline veins from the yolk-sack. From the outer surface of 

 the true chorion (fig. 147, 5, d, 148) villi grow out and fit into 

 crypts or depressions which have in the meantime made their 



1 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. 



