THE HORMONES IN HUMAN REPRODUCTION 



Fig. 28. Enlargement of the human uterus during pregnancy. At v, 

 virginal uterus; the large hatched area is the pregnant uterus, at full 

 term, drawn to same scale, approximately % natural size. Adapted from 

 a figure by Stieve. 



task of the ovary is not yet over, but its functions are in large 

 part to be taken up, reinforced, and even superseded by a new 

 organ of internal secretion, the placenta. The human placenta 

 is an object of considerable size, as shown in the fine old 

 engraving from Casserius (Plate XX). When fully developed 

 it is about 18 centimeters (7.5 inches) in diameter, and weighs 

 500 grams (a little more than one pound) on the average. 

 When in place in the uterus, its structure is like nothing else 

 so much as the matted roots of a tree planted in a tub. The 

 roots simulate the villi of the placenta, which carry within 

 their slender strands the finest branches of the blood vessels 

 coming from the infant in the umbilical cord. Through these 

 delicate vessels the blood of the infant circulates in a constant 

 and rapid stream. The tub in which we imagine the tree 

 planted simulates the pool which the embryo has excavated 

 for itself in the wall of the mother's uterus (Fig. 14, p. 58). 



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