MAMMALIA. 245 



wall, in relation to the blastodermic vesicle, are usually distinguished ; and 

 since the superficial parts of all of these are thrown off with the afterbirth, 

 each of them is called a decidua. They are represented at a somewhartater 

 stage in fig. 151. There is (i) the part of the wall reflected over the blasto- 

 dermic vesicle, called the decidua reflexa (dr) (2) the part of the wall 

 forming the area round which the reflexa is inserted, called the decidua 

 serotina (ds) ; (3) the general wall of the uterus, not related to the embryo, 

 called the decidua vera (du). 



The decidua reflexa and serotina together envelop the chorion, the 

 processes of which fit into crypts in them. At this period both of them are 

 highly and nearly uniformly vascular. The general cavity of the uterus is to a 

 large extent obliterated by the ovum, but still persists as a space filled with 

 mucus, between the decidua reflexa and the decidua vera. 



The changes which ensue from this period onwards are fully known. 

 The amnion continues to dilate (its cavity being intensely filled with amnio- 

 tic fluid) till it comes very close to the chorion (fig. 151, am} ; from which, 



FIG. 151. DIAGRAMMATIC SECTION OF PREGNANT HUMAN UTERUS WITH 

 CONTAINED FOCTUS. (From Huxley after Longet.) 



al. allantoic stalk; nb. umbilical vesicle; am. amnion; ch. chorion; ds. decidua 

 serotina; du. decidua vera; dr. decidua reflexa; /.Fallopian tube; c. cervix uteri; 

 . uterus; z. fretal villi of tnie placenta; z. villi of non-placental part of chorion. 



however, it remains separated by a layer of gelatinous tissue. The villi of 

 the chorion in the region covered by the decidua reflexa, gradually cease to 

 be vascular, and partially atrophy, but in the region in contact with the 

 decidua serotina increase and become more vascular and more arborescent 

 (fig. 151, z). The former region becomes known as the chorion laeve, and 

 the latter as the chorion frondosum. The chorion frondosum, together 

 with the decidua serotina, gives rise to the placenta. 



