1514 THE PLACENTA. [BOOK iv. 



which is called the decidua serotina. In the decidua vera gene- 

 rally, and in the decidua reflexa, the hypertrophy having reached 

 a certain stage ceases, or even gives place to a retrograde process. 

 This change, which may be said to occur at about the fifth month, 

 is most marked and begins earliest in the decidua reflexa, which 

 is soon reduced to a mere membrane, the glands gradually dis- 

 appearing ; later on the decidua vera, though it continues to grow 

 with the expanding uterus, becomes much altered in its more 

 superficial portions. In the decidua serotina, on the other hand, 

 the changes in the mucous membrane which are at first hardly 

 more than those of mere enlargement or hypertrophy, assume new 

 characters and lead to a special union between maternal tissues 

 and tissues belonging to the growing embryo, a union which gives 

 rise to the structure known as the placenta or ' after-birth.' 



951. During the development of the ovum while some of 

 the cells, arising by cell-division from the primordial cell, become 

 the embryo proper, others form the appendages of the embryo ; to 

 the latter belongs the double bag which encloses the embryo, and 

 which consists of an inner bag, the true amnion, and an outer 

 bag, the false amnion. The latter over the whole of its surface is 

 in contact with the decidua, and develops a number of branched 

 villi, consisting, like the rest of the membrane, of an epithelium 

 (epiblast) resting on a dermic (mesoblastic) basis ; these villi are 

 imbedded in or applied to the decidual surface. The false amnion, 

 bearing villi, often called the chorion, is at first devoid of blood 

 vessels ; but a diverticulum of the hinder part of the developing 

 alimentary canal of the embryo, called the allantois, grows out 

 rapidly into the space (containing fluid) between the false and the 

 true amnion, and soon applies itself to the former. As it grows, two 

 arteries, continuations of the primitive aorta, the allantoic 

 arteries, subsequently called umbilical arteries, make their ap- 

 pearance. These carry the blood of the embryo to the villi of the 

 chorion; from thence it is returned at first to two veins, but 

 ultimately to a single vein running in company with the umbilical 

 arteries, and called the umbilical vein. 



At first all the villi over the whole surface of the chorion 

 except at two opposite poles are thus supplied with blood, but 

 ultimately the supply is restricted to that part of the chorion 

 which is applied to the decidua serotina. Here the villi become 

 developed into large and conspicuous vascular tufts, whereas over 

 the rest of the chorion they soon atrophy. 



The decidua serotina at first resembles the rest of the decidua 

 in consisting of enlarged and tortuous glands with hypertrophied 

 dermic tissue between them ; and as in the rest of the decidua, 

 while the changes in the basal zone are relatively speaking not 

 great, the middle portions of the glands become a zone of spongy 

 texture. But this primary condition, in which the decidua sero- 

 tina may still be recognized as a mucous membrane, with its 



