UMBILICAL CORD 809 



This change begins at about the end of the second month, and the pla- 

 centa becomes distinctly limited at about the end of the third month. 



It must be remembered that as the changes go on which result in 

 the formation of the permanent chorion and the limitation of the foetal 

 portion of the placenta, the formation of the umbilical vesicle and the 

 enlargement of the amnion also are progressing. The amnion is grad- 

 ually distended by the increase in the quantity of amniotic liquid. It 

 reaches the internal surface of the chorion at about the end of the fourth 

 month, extends over the umbilical cord to form its external covering, in- 

 cluding the cord of the umbilical vesicle, and the umbilical vesicle itself 

 lies in the gelatinous matter between the two membranes. 



At about the beginning of the fifth month the ovum is constituted as 

 follows : 



The foetus floats freely in the amniotic liquid, attached to the pla- 

 centa by the umbilical cord ; the chorion presents a highly vascular, 

 thickened and villous portion, the foetal portion of the placenta; the 

 rest of the chorion is a simple membrane, without villi and without 

 bloodvessels ; the amnion lines the internal surface of the chorion and 

 forms, also, the external covering of the umbilical cord ; the umbilical 

 vesicle has become atrophied and has lost its vascularity ; the hernia at 

 the point of connection of the umbilical vesicle with the intestine of the 

 foetus has closed; and finally the foetus has undergone considerable 

 development. 



Umbilical Cord. From the description given of the mode of devel- 

 opment of the chorion and the amnion, it is evident that the umbilical 

 cord is nothing more than the pedicle which connects the embryo with 

 that portion of the chorion which enters into the structure of the placenta. 

 It is, indeed, a process of the allantois, in which the vessels have become 

 the most important structures. The cord is distinct at about the end of 

 the first month ; and as development advances, the vessels consist of 

 two arteries coming from the body of the foetus, which are twisted 

 usually from left to right, around the single umbilical vein. In addition 

 to the spiral turns of the arteries around the vein, the entire cord may 

 be more or less twisted, probably from the movements of the foetus. 



The fully-developed cord extends from the umbilicus of the foetus to 

 the central portion of the placenta, in which its insertion usually is 

 oblique ; although it may be inserted at other points, and even outside 

 of the border of the placenta, its vessels penetrating this organ from the 

 side. Its usual length, which varies considerably, is about twenty inches 

 (50.8 centimeters). It has been observed as long as sixty (152.4 centime- 

 ters), and as short as seven inches (17.8 centimeters). When the cord is 

 very long, it sometimes presents knots or it may be wound around the 



