FCETAL MEMBRANES 



823 



am- 

 " while 



the endoderm of the embryo, the mesoblast or mesoderm 

 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 mesoderm, carrying with it a single sheet of 

 ectoderm, 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 " 

 nion proper, 

 the outer layers 

 also unite to form 

 a layer lying in- 

 ternally to the ec- 

 todermal blastocyst 

 wall — and termed 

 by Sir William 

 Turner the siibzonal 

 membrane. The 

 folds of amnion are 

 continued, as the 

 diagram shows, 

 ventrally as well as 

 dorsally, so that the 

 subzonal mem- 

 brane surrounds 

 the embryo beneath 

 the blastocyst wall, 

 while a splanchnic 

 layer of mesoderm 



grows round about the endodermal yolk-sac. The space 

 between the two layers of mesoderm is continuous with 

 the body cavity of the embryo. The ectodermal outer 

 wall or trophoblast, and the mesodermal subzonal mem- 

 brane, are included in Hubrecht's term — diplotrophoblast. 

 (5) From the hind-wall of the gut there grows out an 

 endodermal sac, the allantois, insinuating itself and 

 spreading out in the space between the two layers of 

 mesoderm. As an outgrowth of the gut, homologous 

 with the bladder of the frog, the allantois is Hned by 



Fig. 496. 



-Diagram of foetal membranes. 

 — After Turner. 



E., Embryo ; H., gut lined by endoderm, dotted — 

 the dark is mesoderm ; U.V., umbilical vesicle or 

 yolk-sac; A.C., amniotic cavity; am., amnion 

 proper ; sz., subzonal membrane ; AL.C, allantoic 

 cavity ; al., allantois ; zp. may be here taken to 

 represent the early ectodermal trophoblast. The 

 figure does not show that the amnion folds consist 

 of both ectoderm and mesoderm. 



