FORMATION OF THE AMNION. 803 



and the union of these two structures forms the placenta. The foetal portion 

 of the placenta is connected with the fcetus, by the vessels of the umbilical 

 cord, and the maternal portion is connected with the great uterine sinuses. 



The external covering of the ovum, during the first stage of its develop- 

 ment, is the vitelline membrane. As the ovum is received into the uterus, 

 the vitelline membrane develops upon its surface little villosities, which are 

 non-vascular and are formed of amorphous matter with granules. These are 

 the first villosities of the ovum, and they assist in fixing the egg in the uter- 

 ine cavity. They are not permanent, they do not become developed into the 

 vascular villosities of the chorion and they disappear as the true membranes 

 of the embryon are developed from the blastodermic layers. The vitelline 

 membrane disappears soon after the passage of the ovum into the uterus, 

 when it is replaced by the amnion. 



Formation of the Amnion. As the ovum advances in its development, it 

 is observed that a portion of the blastoderm becomes thickened, forming the 

 epiblast, the two layers of the mesoblast and the hypoblast. At about the 

 time when this thickening begins, a fold of the epiblast and of the external 

 layer of the mesoblast makes its appearance, which surrounds the thickened 

 portion and is most prominent at the cephalic and the caudal extremity of 

 the furrow for the neural canal. This fold increases in extent as develop- 

 ment advances, passes over the dorsal surface of the embryon and finally 

 meets so as to enclose the embryon completely. At a certain period of the 

 development of the amnion, this membrane consists of .an external layer, 

 formed of the external layer of the fold, and an internal layer ; and the point 

 of union of the two layers, or the point of meeting of the fold, is marked by 

 a membranous septum. 



The two amniotic layers are formed in the way just described, and a com- 

 plete separation finally takes place, by a disappearance of the septum formed 

 by the meeting of the folds over the back of the embryon. This process 

 occupies four or five days, in the human ovum. The point where the folds 

 meet is called the amniotic umbilicus. When the amnion is thus completely 

 formed, the vitelline #nembrane has been encroached upon by the external 

 amniotic layer and disappears, leaving this layer of the amnion as the external 

 covering of the ovum. At this time there is a growth of villosities upon the 

 surface of the external amniotic layer, which, like the villosities of the vitel- 

 line membrane, are not vascular. 



Soon after the development of the amnion the allantois is formed. This 

 membrane is vascular. It encroaches upon- and takes the place of the external 

 amniotic membrane, and is covered with hollow villi, which take the place 

 of the villi of the amnion. Over a certain portion of the membrane the villi 

 are permanent. The mode of development of the amnion is illustrated by 

 the diagrammatic Fig. 295. This figure illustrates the formation of the 

 amnion, the umbilical vesicle and the allantois. The last two structures are 

 derived from the hypoblast and the internal layer of the mesoblast. 



When the allantois has become the chorion, or the external membrane of 

 the ovum, having taken the place of the external layer of the amnion, the 



