908 GENERATION. 



from the margin of this depression, which extends around and encloses the ovum, pre- 

 senting a spot where the final closure takes place, called the decidual umbilicus. 



We have given the recent views of Reichert thus fully, for the reason that they are 

 based upon the study of a remarkably young ovum and appear to be more exact and 

 definite than any observation hitherto recorded ; and we shall adopt this description as 

 representing the early stages of the formation of the membranee deciduse. 



According to Reichert, the ovum is completely enclosed at the twelfth or the thir- 

 teenth day. The mucous membrane lining the uterus becomes the decidua vera, and the 

 border from which the new growth is formed which covers the ovum is the boundary 

 between this and the decidua reflexa. The new growth, springing from this border, en- 

 velops the ovum completely and is called the decidua reflexa; and, in this membrane, 

 there is no trace of the uterine tubules. 



As development advances, a portion of the decidua vera the description of which we 

 reserve for the present undergoes development into the maternal portion of the placenta. 

 The rest of the decidua vera becomes extended, loses its vessels and glands, and is reduced 

 to the condition of a simple membrane. The cylindrical epithelial cells of the mucous 

 membrane of the body of the uterus, soon after fecundation, become gradually exfoliated, 

 and their place is supplied by flattened epithelial scales, of the pavement-variety. This 

 change is effected at from the sixth to the eighth week, and the pavement-cells are then 

 found covering both the decidua vera and the reflexa. The epithelium of the cervix 

 retains its cylindrical character, but most of the cells lose their cilia. 



During the first periods of utero-gestation, the two layers of decidua are separated by 

 a small amount of an albuminous and sometimes a sanguinolent fluid ; but this disappears 

 at about the end of the fourth month, and the membranes then come in contact with each 

 other. They soon become so closely adherent as to form a single membrane, which is in 

 contact with the chorion. Sometimes, at full term, the membranes of the foetus can be 

 separated from the decidua; but frequently all of the different layers are closely adherent 

 to each other. 



The changes we have just described are not participated in by the mucous membrane 

 of the neck of the uterus. The glands in this situation secrete a semisolid, transparent, 

 viscid mucus, which closes the os and is sometimes called the uterine plug. 



Toward the fourth month, a very delicate, soft, homogeneous layer appears over the 

 muscular fibres of the uterus, beneath the decidua vera, which is the beginning of a new 

 mucous membrane. This is developed very gradually, and the membrane is completely 

 restored about two months after parturition. 



Development and Structure of the Placenta. In describing the formation of the mem- 

 branas decidusB and of the chorion, we have necessarily hinted at the mode of development 

 of the placenta. Although there is considerable difference of opinion among anatomists 

 with regard to the exact relations between the vessels of the mother and of the foetus in 

 utero, it is admitted by all that the foetus derives its nourishment from the maternal blood, 

 and that the placenta is, in addition, a respiratory organ. Reasoning from what we 

 should consider to be the requirements of the foetus, it would be natural to suppose that 

 the foetal vessels are bathed in maternal blood ; and it is certain that the two sets of 

 vessels have no direct communication with each other. It is also well known that the 

 foetus has an independent circulation, its heart beating about twice as fast as the heart of 

 the mother. In our description of the placenta, we shall first give the views which we 

 conceive to be correct, and then advance the facts and arguments by which these views 

 are apparently supported. 



Beginning with the first development of the placenta, the observation which we have 

 quoted from Reichert, in which, it will be remembered, the tufts of the foetal chorion 

 were actually drawn out of the tubules of the uterine mucous membrane, seems to de- 

 monstrate beyond question the fact of penetration of the villi of the chorion into the 



