356 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 



blastodermic vesicle, are usually distinguished; and since the 

 superficial parts of all of these are thrown off with the after- birth, 

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

 somewhat later stage in Fig. 117. There is (1) the part of the 

 wall reflected over the blastodermic 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 

 (Fig. 114. 5), 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 tensely 

 filled with amniotic fluid) till it comes very close to the chorion 

 (Fig. 117, am); from which, 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. 117, z). The former region becomes known as 

 the chorion Iceve, and the latter as the chorion frondosum. The 

 chorion frondosum, together with the decidua serotina, gives rise 

 to the placenta. 



The umbilical vesicle (Fig. 117, rib\ although it becomes 

 greatly reduced in size and flattened, persists in a recognisable 

 form till the time of birth. 



The decidua reflexa, by the disappearance of the vessels in the 

 chorion Iseve, becomes non-vascular. Its tissue and that of the 

 decidua vera undergo changes which we do not propose to 

 describe here ; it ultimately fuses on the one hand with the 

 chorion, and on the other with the decidua vera. The mem- 

 brane resulting from its fusion with the latter structure becomes 

 thinner and thinner as pregnancy advances, and is reduced to a 

 thin layer at the time of birth. 



