THE CH ORION AND AM N ION. 117 



The Chorion and Amnion. 



These are two membranes which always surround the embryo, the chorion 

 being the outer, the amnion the inner, membrane of the two. Morphologically, they 

 are modifications of the extra-embryonic somatopleure. The accompanying diagrams 

 render this clear. In figure 29, we see that the cavity of the mesoderm, coe, has ex- 

 tended completely around the yolk. There is a layer of mesoderm, represented by 

 a dotted line, on the outside of this cavity, which joins with the overlying ecto- 

 derm, represented in the diagram by a continuous line, to constitute the somato- 

 pleure, Som. In figure 45, we see the somatopleure folded up on the dorsal side of 

 the embryo; the leaf of the fold nearest the embryo is the anlage of the amnion, 

 aw; the rest of the extra-embryonic somatopleure is the anlage of the chorion' In 

 the second figure of the diagram, the two folds have met above the embryo and 

 united, thus making a closed inner amniotic and a closed outer chorionic envelope. 

 The actual appearances of two such stages as in the diagram are illustrated by 

 figures 38 and 47. By this account we learn that the two envelopes are produced by 

 a folding of the somatopleure. 



When we come to study the development of mammals in detail, we discover 

 that there are many remarkable variations in the early development of the amnion 

 of which no general explanation is yet possible; but inasmuch as the folding pro- 

 cess is the only one in Sauropsida, and also occurs in many mammals of different 

 classes, it is generally assumed to be the primitive method. 



In man the development of the chorion and amnion differs extremely from the 

 scheme given above. It is described as accurately as our present knowledge per- 

 mits in Chapter III. 



However developed, the fetal envelopes present certain constant characteristics: 

 both consist of ectoderm and mesoderm, but in the case of the amnion the ecto- 

 derm is turned toward the embryo, whereas the chorionic ectoderm faces the out- 

 side. The cavity between the amnion and the embryo becomes filled with the 

 amniotic fluid, which serves as an important mechanical protection to the develop- 

 ing embryo. It is through the chorionic ectoderm only that the ovum can come 

 into actual contact with the walls of the uterus. It is the chorion alone which is 

 concerned in the formation of the true placenta (compare Chapter VII). 



The amnion is a thin, pellucid, non- vascular membrane; the chorion is thicker, 

 more nearly opaque, and has in man and all nearly related animals a highly de-. 

 veloped vascular system. 



