802 EMBRYOLOGY 



the allantois, which is vascular. The two layers of decidua are formed 

 from the mucous membrane of the uterus. At a certain part of the 

 uterus, a vascular connection is established between the mucous mem- 

 brane and the allantois, and the union of these two structures forms the 

 placenta. The foetal portion of the placenta is connected with the foetus 

 by the vessels of the umbilical cord, and the maternal portion is con- 

 nected with the great uterine sinuses. 



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

 development, is the vitelline membrane. As the ovum is received into 

 the uterus, the vitelline membrane develops on its surface little villosi- 

 ties, which are non-vascular and formed of amorphous matter with 

 granules. These are the first villosities of the ovum, and they assist in 

 fixing the egg in the uterine 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 embryo 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 develop- 

 ment, 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 extremities of the furrow for the neural canal. This fold 

 increases in extent as development advances, passes over the dorsal 

 surface of the embryo and finally meets so as to enclose the embryo 

 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 membra- 

 nous septum. 



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

 a complete separation finally takes place, by a disappearance of the 

 septum formed by the meeting of the folds over the back of the 

 embryo. 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 membrane 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 on the surface of the external amni- 

 otic layer, which, like the villosities of the vitelline membrane, are not 

 vascular. 



