906 GENERATION. 



without blood-vessels; the amnion lines the internal surface of the chorion and also 

 forms 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 um- 

 bilical vesicle with the intestine of the fcetus has closed ; and, finally, the foetus has under- 

 gone a considerable degree of development. 



It now remains for us to study the structure of the umbilical cord, the membranes 

 formed from the mucous membrane of the uterus, or the membransQ decidua), and the 

 mode of development and the structure of the placenta. 



Umbilical Cord. From the description we have given of the mode of development 

 of the chorion and the amnion, it is evident that the umbilical cord is nothing more than 

 the pedicle which connects the embryon 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 eventually 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 fcetus, which are usually twisted from left to right 

 around the single umbilical vein. In addition to the spiral turns of the arteries around 

 the veins, the entire cord may be more or less twisted, probably from the movements of 

 the fcetus. 



The fully-developed cord extends from the umbilicus of the fcetus to the central por- 

 tion of the placenta, in which its insertion is usually oblique ; though 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 very considerably, is about 

 twenty inches. It has been observed as long as sixty, and as short as seven inches. 

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

 neck, the body, or any of the members of the fcetus ; and this can only be accounted for 

 by the movements of the fcetus in utero. 



The external covering of the cord is a process of the amnion, which, as it extends 

 over the vessels, includes a gelatinous substance (the gelatine of Wharton) which sur- 

 rounds the vessels and protects them from compression. This gelatinous substance is 

 identical with the so-called membrana intermedia, or the substance included between the 

 amnion and the chorion. The entire cord, covered with the gelatine of Wharton and 

 the amnion, is usually about the size of the little finger. According to Eobin, the nor- 

 mal cord will sustain a weight of from ten pounds and ten ounces to twelve pounds and 

 twelve ounces avoirdupois. As the amniotic fluid accumulates and distends the amniotic 

 membrane, it becomes more and more closely applied to the cord. This pressure extends 

 from the placental attachment of the cord toward the fcetus and gradually forces into 

 the abdomen of the foetus the loop of intestine, which, in the early periods of intra- 

 uterine life, forms an umbilical hernia. 



It is generally stated by writers upon embryology that the vessels of the cord present 

 no valves ; but recent observations have demonstrated the presence of semilunar folds, 

 both in the vein and in the arteries. These are simple inversions of the walls of the ves- 

 sels ; and they do not exist in pairs, nor do they seem to influence the current of blood. 

 In the arteries, these folds are situated at intervals of from half an inch to two inches, 

 and they are more abundant where the vessels are very contorted. In the vein, the folds 

 are most abundant near the placenta ; they are very irregularly placed, and, in a length of 

 four inches, fifteen folds were found. It is not apparent that these folds have any physio- 

 logical importance. 



As the allantois is developed, it presents, in the early stages of its formation, three 

 portions ; an external portion, which becomes the chorion, an internal portion, enclosed 

 in the body of the embryon, and an intermediate portion. The intermediate portion, as 

 we have seen, becomes the umbilical cord. As the umbilicus of the foetus closes around 

 the cord, it shuts off a portion of the allantois contained in the abdominal cavity, which 



