THE HUMAN FCETAL MEMBRANES. 35 



foetal structures. The mucous membrane of the entire uterine cavity, in many of the 

 higher mammals, suffers profound change, and before the end of gestation becomes 

 inseparably attached to the chorion even in its extent beyond the placental area ; in 

 such animals the fused uterine and chorionic tissue constitute the decidiicB which, 

 lined internally by the closely applied amnion, form the membranous envelope en- 

 closing the foetus. After rupture consequent upon the e.xpulsion of the foetus at the 

 termination of pregnancy, the deciduae, including the specialized placental portion, 

 are separated from the uterine wall and expelled as the membranes and the placenta 

 which are known collectively as the after-birth. 



The foregoing sketch of the general development of the fcetal membranes in 

 the higher, mammals must be now supplemented by consideration of the peculiarities 

 encountered in the development of these structures in man. 



THE HUMAN FCETAL MEMBRANES. 



The young human embryo is distinguished by the very early formation of the 

 amniotic ca\ity, by the precocious development of the mesoblast and extra- 

 embryonic coelom, by the presence of the body-stalk and by the great thickening of 

 the trophoblast. It must be remembered, in considering the formation of the human 

 foetal membranes, that the earliest stages of development, to wit, fertilization, 

 segmentation, the formation of the blastodermic vesicle, the earliest differentiation of 

 the embryonic area and the formation of the amniotic cavity ha\e not yet been 

 observed on human specimens. Our knowledge of these processes is derived from a 

 study of some of the lower types ; beyond these very early stages, however, the 

 conditions in the human embryo have been subject to direct study. 



The Human Amnion, Amniotic Cavity and Allantois. — The accompany- 

 ing diagrams (Fig. 43) will serve to illustrate the process of formation of the foetal 

 membranes in man. Of these five diagrams, A alone is purely hypothetical with 

 reference to the human embryo. In diagram A the amniotic cavity is already 

 indicated as a small cleft between the embryonic area below and a covering layer of 

 cells above continuous with the trophoblast. This layer, the tropJwblast, forms the 

 outer covering of the entire vesicle. It is presumably already thickened at as early 

 a stage as this diagram represents. Presumably also the surface of the trophoblast 

 shows irregularities, for this tissue it is which comes into direct contact with the 

 uterine mucous membrane and which, by its activities, forces its way into the 

 maternal decidua. This latter process is known as implantation, a process which 

 supposedly is taking place, if not completed, at about the stage of this diagram. 

 Whether the trophoblastic layer in man is originally a thin single sheet of cells, as 

 for instance is the case in the rabbit, or whether it is from the beginning thickened, 

 we do not know. Certainly the thickened condition appears at a very early stage. 

 The embryonic area shows the embryonic ectoblast proper, which is of small extent ; 

 this ectoblast being so distinguished from the trophoblastic ectoblast. The ento- 

 blast beneath is represented as already arranged in the form of a sac. Between the 

 entoblast and ectoblast the mesoblast has made its appearance. It will be noted 

 that in the diagram the entoblastic sac is much smaller than the outer trophoblastic 

 vesicle. We do not know that this is really the condition when the entoblastic sac 

 is first formed or only appears in conjunction with the great development of the extra 

 embryonic coelom in the mesoblast. It is certainly not unreasonable to suppose that 

 the former case is the true one. 



The early appearance of the amniotic cavity is to be explained in this way. 

 After the blastodermic vesicle has reached the stacje when the inner cell mass is 

 attached to one point on the inner surface of the trophoblast, the formation of a 

 cavity occurs in the region of the inner mass. This cavity, at first very small, 

 has below it the cells of the inner mass, which soon become arranged into the 

 two primary germ layers of the embryonic area, ectoblast and entoblast, while 

 above the cavity is a layer of cells continuous with the trophoblast. Such a 

 method of formation of the amniotic cavity has been observed in some of the 

 lower forms, for instance, in a lemur by Hubrecht, and since the earliest human 

 embryo accurately studied shows a completely closed amniotic cavity, while in 



