COVERINGS OF THE OVUM. 361 



Formation of the Membranes. 



The brief description we have just given of the forma- 

 tion of the blastodermic layers seemed necessary as an intro- 

 duction to the study of the membranes ; and we shall defer, 

 for the present, the description of their development into the 

 different parts of the embryon. 



In the mammalia, a portion of the blastoderm is devel- 

 oped into membranes, by which a communication and union 

 are established between the ovum and the mucous membrane 

 of the uterus. From the ovum, are developed two membranes, 

 one non-vascular, the amnion, and another vascular, the allan- 

 tois. From the mucous membrane of the uterus, are devel- 

 oped the two layers of the decidua. At a certain part of the 

 uterus, a vascular connection is established between the mu- 

 cous membrane and the allantois, and the union of these two 

 structures forms the placenta. The foetal portion of the pla- 

 centa is connected with the foetus by the vessels of the um- 

 bilical cord ; and the maternal portion is connected with the 

 great uterine sinuses. Development takes place from material 

 supplied 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. According 

 to Coste, as the ovum is received into the uterus, the vitelline 

 membrane develops upon its surface little villosities, 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, 

 do not become developed into the vascular villosities of the 

 chorion, and disappear as the true membranes of the embryoil 

 are developed from the blastodermic layers. 1 It is probable 

 that the vitelline membrane disappears about the fourth or 

 fifth day, when it is replaced by the amnion. 



Formation of the Amnion? As the ovum advances in its 



1 COSTE, Developpement des corps organises, Paris, 1847, tome i., p. 82. 



8 In the literature pertaining to the subject of development, there is no de- 



