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A TEXTBOOK OF PHYSIOLOGY 



in thickness, forming itself into two layers a compact superficial 

 and a deep spongy layer. It is now known as the decidua, and is 

 divided into three portions the decidua basalis, or serotina, upon 

 which the ovum rests ; the decidua reflexa, or capsularis, which 

 encloses the embedded ovurn; and the decidua vera, the remaining 

 portion of the mucous membrane not in contact with the ovum 

 (Figs. 482, 483). At first a space the decidual space between the 

 two latter parts represents the remains of the true uterine cavity. 

 These eventually come into contact, and fuse in the human subject 

 in the fourth month of pregnancy. 



MUCOUS 

 MEMBRANE 

 Of 



UTERUS 



FIG. 48). SHOWING ORIGIN OF THE PRIMITIVE CCELOM, THE MESOBLAST, AMI CAVITY 

 OF THE AMNION DURING DEVELOPMENT OF THE HITMAN OVUM. (Keith, after 

 I. H. Bryce.) 



From the developing embryo, two membranes are formed the 

 chorion and the amnion (Fig. 482). The chorion is the outer layer. 

 It early divides into two an outer fused mass of cells, or syncytium ; 

 an inner layer of cells, or Langhans' layer. During the first six weeks 

 the whole chorion becomes covered with vascular villi. These, how- 

 ever, soon disappear except in the region of the decidua basalis, where 

 the ovum is attached. Here is formed the chorion frondosum, its villi 

 and the decidua basalis fuse together, and form the placenta. 



Within the chorion is the closed sac the amnion filled with 

 fluid, in which the embryo is bathed. 



The placenta is formed as a separate organ about the third month 

 of pregnancy, gradually increasing in size according to the foetal needs 

 until full term. It is formed by a fusion of the decidua basalis and the 

 chorion frondosum (Fig. 484). Blood-sinuses become developed in both 

 the maternal and foetal portions, so that the maternal and foetal blood 



