72 HUMAN EMBRYOS AND FETAL MEMBRANES 



portant, not only as an organ of respiration and excretion, but as an organ 

 of nutrition. Through its vessels it has taken on a function belonging to 

 the yolk sac in birds, and we now see why the yolk sac becomes a rudimen- 

 tary structure in the higher mammals. Excreta from the embryonic 

 kidneys are passed into the cavity of the allantois which is relatively large. 

 The name is derived from a Greek word meaning sausage-like, from its 

 form in some animals. The chorion is important only as it brings the 

 allantois into close relation to the uterine wall, but in man we shall see 

 that it plays a more important role. 



THE UMBILICAL CORD 



Pig Embryos. -In their early development, the relation of the amnion, 

 allantois, and yolk sac to each other and to the embryo is much the same 

 as in the chick of five days (Fig. 71). With the increase in size of the 

 embryo, however, the somatopleure in the region of the attachment of the 

 amnion grows ventrad (Fig. 70 D). As a result, it is carried downward 

 about the yolk sac and allantois, forming the umbilical cord (cf. Fig. 

 241). Thus, in a pig embryo 10 to 12 mm. long, the amnion is attached at 

 a circular line about these structures some distance from the body of the 

 embryo (cf. Fig. 119). The coelom at first extends ventrad into the cord, 

 but later the mesodermal layers of amnion, yolk stalk, and allantois 

 fuse and form a solid cord of tissue. This is the umbilical cord of fetal 

 life and its point of attachment to the body is the umbilicus, or navel. 

 The cord is covered by a layer of ectoderm continuous with that of the 

 amnion and of the embryo, and contains, embedded in a mesenchymal 

 (mucous) tissue: (i) the yolk stalk and (in early stages) its vitelline 

 vessels; (2) the allantoic stalk; (3) the allantoic vessels. These latter, 

 two arteries and a single large vein, are termed, from their position, the 

 umbilical vessels. At certain stages (Figs. 122 and 123) the gut normally 

 extends into the coelom of the cord, forming an umbilical hernia. Later, it 

 returns to the coelom of the embryo and the cavity of the cord disappears. 

 The umbilical cord of the pig is very short. 



The Human Umbilical Cord. -This develops like that of the pig and may 

 attain a length of more than 50 cm. It becomes spirally twisted, just 

 how is not known. In embryos from 10 to 40 mm. long he gut extends 

 into the ccelom of the cord (Figs. 179 and 180). At the 42 mm. stage, 

 the gut returns to the ccelom of the body. The mucous tissue peculiar to 

 the cord arises from mesenchyme. It contains no capillaries and no nerves, 

 but embedded in it are the large umbilical vein, the two arteries, the allan- 

 tois, and the yolk stalk. The umbilical cord may become wound about 

 the neck of the fetus, causing its death and abortion, or by coiling about 

 the extremities it may lead to their atrophy or amputation. 



