912 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



value, and the minute quantity of urea is perhaps indicative of an unimport- 

 ant excretory function. As growth proceeds, the amnion expands and becomes 

 loosely attached to the outer fetal membrane, the chorion. 



The chorion (Fig. 314), or false amnion, is formed simultaneously with the 

 true amnion, and like it from somatopleure. It is a thickened vascular mem- 

 brane, completely surrounding the amnion with the contained embryo. Be- 

 tween it and the amnion there is at first a considerable space, traversed by the 

 umbilical cord and filled with the chorionic fluid (which is probably of the 

 same general nature as the amniotic fluid). But later this space is obliterated 

 by the enlargement of the amnion. Externally the chorion presents, at first, 

 a* shaggy appearance due to the existence of very numerous columnar pro- 

 cesses, called mill, extending outward in all directions and joining by their 

 tips the decidua serotina and the decidua reflexa. Later the villi are aborted 

 except in the region of the serotina, where they become more prominent and 

 constitute an important part of the placenta. The blood-vessels of the chorion 

 are fetal vessels coming from the embryonic structure, the allantois. They 

 comprise the branches and uniting capillaries of the two allantoic or umbilical 

 arteries, and the one (at first two) allantoic or umbilical vein. They are 

 especially well developed within the villi. As growth proceeds, the chorion 

 comes into close contact with the decidua reflexa, and, as the latter disappears, 

 with the decidua vera ; this portion of it is called chorion Iceve. In the region 

 of the decidua serotina it enters into the formation of the placenta, and is 

 here called chorion frondosum. 



The Placenta. The placenta (Fig. 314), or organ of attachment of mother 

 and fetus, is a disk-shaped body, approximately 20 centimeters (7-8 inches) in 

 diameter, attached to the inner surface of the uterine wall, usually either upon 

 the dorsal or the ventral side, more frequently upon the former, and connected 

 by the umbilical cord with the navel of the fetus. It consists of a maternal 

 part, the modified decidua serotina, and a fetal part, the modified chorion, inti- 

 mately united together. The modifications of the serotina consist of a degen- 

 eration of the superficial layers of the mucosa, especially of the epithelium 

 and the glands, and the development of very large irregular sinuses at the 

 surface, into which the uterine arteries and veins appear freely to open. It 

 should be said that it is a disputed question among histologists whether the 

 sinuses are maternal or fetal in origin, or really spaces between maternal and 

 fetal tissues. It is also disputed whether they actually contain blood or only 

 fluid from the surrounding tissues ; the former has by far the weight of evi- 

 dence in its favor and is the prevailing view. The modifications of the chorion 

 consist of a great increase in length and complexity of branching of the villi, 

 a great development of their contained blood-vessels, and a firm attachment 

 of their tips to the uneven surface of the serotina, so that their branches come 

 to float freely within the uterine sinuses and to be bathed in uterine blood 

 (Fig. 315). The analogy between the mammalian placenta I villi and the gills 

 of a fish, also highly vascular and floating in liquid, is striking. We shall 

 see later that the analogy is not only morphological, but also physiological, 



