FORMATION OF THE MEMBRANES. 901 



amnion, and another vascular, the allantois. From the mucous membrane of the uterus, 

 are developed. the two layers of the decidua. At a certain part of the uterus, a vascular 

 connection is established between the mucous membrane and the allantois, and the union 

 of these two structures forms the placenta. The foetal portion of the placenta is con- 

 nected with the foetus by the vessels of the umbilical cord ; and the maternal portion is 

 connected with the great uterine sinuses. Development takes place from material sup- 

 plied to the foetus by the blood of the mother. 



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 upon its surface little villosities, which are non- vascular and are formed of amor- 

 phous 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 embryon are developed from the blastodermic layers. It is probable 

 that the vitelline membrane disappears about the fourth or fifth day, when it is replaced 

 by the anmion. 



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

 that a portion of the blastoderm becomes thickened, to form the first trace of the embryon. 

 At this portion, where the body of the embryon subsequently makes its appearance, as 

 we have already seen, we have the external layer, the internal layer, and a thick, inter- 

 mediate layer of cells, which are developed from the opposite surfaces of the external and 

 the internal layer, called the middle layer. At nearly the time when this thickening begins, 

 a fold of the external layer makes its appearance, surrounding the thickened portion, and 

 most prominent at the cephalic and the caudal extremity of the furrow for the neural canal. 

 This fold increases in extent as development advances, passes over the dorsal surface of 

 the embryon, and finally meets so as to enclose the embryon completely. We can readily 

 figure to ourselves this process and understand how, at a certain period of the develop- 

 ment of the amnion, this membrane consists of an external layer, formed of the external 

 layer of the fold, and an internal layer ; the point of union of the two layers, or the point 

 of meeting of the fold, being marked by a membranous septum. 



The two amniotic layers are formed in the way that we have 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 embryon. This process occupies four or five 

 days in the human ovum. The point where the folds meet is called the amniotic umbili- 

 cus. 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 villosi- 

 ties 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, encroaches upon and takes the place of the external amniotic membrane, 

 becomes villous, and its villosities take the place of those of the amnion. Over a certain 

 portion of the membrane, the villosities are permanent. The mode of development of the 

 amnion, as we have described it, is illustrated by the diagrammatic Fig. 291. This figure 

 illustrates the formation of the amnion, the umbilical vesicle, and the allantois. The last two 

 structures are not derived from the external blastodermic layer, and they will be described 

 farther on, after we have studied the full development of the amnion and its relations. 



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 structures of the ovum 

 are the following: 1. The chorion, formed of the two layers of the allantois, derived from 

 the internal blastodermic membrane, and penetrated by blood-vessels. 2. The umbilical 

 cord, which connects the embryon with the placental portion of the chorion, and the um- 



