1140 THE REPRODUCTIVE ORGANS 



80 or 85 per cent. There may also be noted a slight increase in the 

 percentage of ammonia. 



The Development of the Placenta. The cytoplasm of the ovum 

 contains a certain amount of nutritive material which, however, does 

 not last for a longer time than its initial period of growth. Hence, a 

 new source of supply must be established as soon -as the ovum has 

 become firmly attached to the maternal tissues. It will be remembered 

 that the defect through which the ovum has entered the uterine 

 decidua, closes soon afterward, the layer of tissue now investing the 

 ovum externally being known as the decidua reflexa, and that lining 

 the substance of the uterus as the decidua vera. Directly under- 

 neath the developing ovum lies the decidua basalis. The latter, 

 together with the enveloping membranes of the ovum, now enters into 

 the formation of a special organ, the placenta, the purpose of which 

 is to effect an interchange of materials between the fetus and the 

 mother. Evidently, this structure arises from a union of certain 

 fetal and maternal tissues, and consists essentially of vascular 

 outgrowths or villi of the chorion of the fetus which become approxi- 

 mated to large blood spaces formed in the decidua basalis of the uterus. 

 Consequently, the blood of the mother does not actually pass into the 

 channels of the fetus, but remains separated from that of the latter by 

 the lining of the blood-vessels and the epithelial layers of the villi of 

 the chorion. 



To begin with, this separation is effected by a single layer of cells of the ectoderm 

 and constitutes the external envelope of the blastodermic vesicle. As soon as the 

 ovum has become firmly lodged upon the decidua, these cells proliferate and project 

 outward in the form of minute finger-like processes or villi, which impart a peculiar 

 fringed appearance to this layer. Each villus, therefore, is situated upon a substra- 

 tum of connective tissue with which it remains connected by a stalk, and consists 

 of an outer epithelial covering and an inner framework of connective tissue. The 

 cells of the former frequently proliferate, forming additional minute buds upon the 

 individual villi. It is also to be noted that the latter are very numerous in that 

 region of the ovum which lies most directly in contact with the basilar decidua. 



Mention should also be made of the amnion which, when fully formed, com- 

 pletely invests the embryo, and eventually comes to lie in close contact with the 

 inner surface of the chorion. The amnion is developed as two layers, an external 

 one consisting of mesoderm and an internal one composed of cuboidal or flattened 

 ectoderm. Aclear fluid then collects between these layers which gradually increases 

 in quantity as pregnancy progresses. Its average amount at term is 600 c.c. and its 

 specific gravity 1.002 to 1.028. It is derived chiefly from the mother's serum by 

 transudation through the amniotic epithelium, 1 but may also contain fetal urine 

 during the last months of pregnancy if the mother's kidneys become defective. 2 

 The function of this fluid is chiefly protective, because it serves to mitigate the force 

 of sudden shocks and to prevent the loss of heat from the fetus by investing its 

 surface with a medium of constant temperature. Secondly, it keeps the developing 

 fetus away from other tissuessothatitcannot become adherentto them. Thirdly, 

 it may supply water as well as albumin to its tissues. Lastly, it serves as the 

 normal dilator of the cervix of the uterus during labor. In this case, it acts as a 



1 Ahlfeld, Zeitschr. fur Geb. und Gynec., Ixix, 1911, 91. 



2 Wolff, in Oppenheimer's Handb. der Bioch., iii, 1910, 709. 



