UMBILICAL COED 79 



both chorion and uterus to be taken up by the blood of the mother. In the same 

 way nutritive substances and oxygen must pass from the maternal blood through 

 these layers to enter the allantoic vessels. This exchange does take place, how- 

 ever, and thus in Ungulates the allantois has becomfc important 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 rudimentary 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 saus- 

 age-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 r61e. 



UMBILICAL CORD 



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. 68). With the increase in size of the embryo, however, the somato- 

 pleure in the region of the attachment of the amnion grows ventrad. As a 

 result, it is carried downward with the ccelom about the yolk-sac and allantois, 

 forming the umbilical cord. 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. The ccelom 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 con- 

 tinuous with that of the amnion and of the embryo and contains, embedded in a 

 mesenchymal (mucous) tissue (1) the yolk-stalk and (in early stages) its vitelline 

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

 and a single large vein, are termed from their position the umbilical vessels. At 

 certain stages (Figs. 117 and 118) the gut normally extends into the ccelom of 

 the cord, forming an umbilical hernia. Later, it returns to the ccelom of the 

 embryo and the cavity of the cord disappears. The umbilical cord of the pig 

 is very short. 



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 mm. to 40 mm. long the gut extends into the ccelom of the 



