842 DEVELOPMENT [CH. LIX. 



At first the villi are small, but, as they project into the decidua 

 capsularis and decidua basalis, they grow rapidly and branch repeatedly. 

 Their function is to obtain nutriment from the uterine tissues. In 

 the higher mammals, including man, they destroy and eat up many 

 of the cells of the decidua, and gases and fluids pass through them 

 from the maternal to the foetal blood, and vice versd. In some 

 mammals, however, they do not destroy the uterine tissues, and in 

 those cases they absorb the uterine milk, which is secreted by the 

 enlarged uterine glands. 



The chorionic villi which penetrate the decidua capsularis gradu- 

 ally disappear as the capsularis fuses with the vera, and is reduced to 

 a thin membrane; but the villi which enter the decidua basalis 

 increase enormously in size and complexity, to form the foetal part of 

 the placenta, and their branches hang free in the interiors of large 

 blood sinuses which are filled with maternal blood (fig. 634). 



Allantois. The allantois is an outgrowth from the ventral portion 

 of the posterior part of the primitive alimentary canal, and it consists of 

 a hollow process of hypoblast covered with mesoblast (fig. 631, 12). In 

 the human embryo it appears at a very early period, before the amnion 

 folds have separated from the chorion, and it conveys the allantoic 

 arteries from the embryo to the chorion, and the allantoic vein from 

 the chorion to the embryo. As development proceeds, and that part 

 of the chorion in contact with the decidua basalis is converted into 

 the foetal part of the placenta (figs. 634 to 637), the allantoic blood- 

 vessels in the chorion gradually disappear except in the placental area 

 where they grow larger till birth. 



At first the allantois is very short, but, as the amnion distends and 

 the embryo passes further and further into the interior of the enlarg- 

 ing ovum, it is elongated into a cord which, together with the remains 

 of the yolk-sac is surrounded and ensheathed by the amnion ; this 

 cord is called the umbilical cord (fig. 634). 



In the human subject that portion of the allantois which lies in 

 the umbilical cord consists entirely of vascular mesoblast, for the 

 hollow pouch of hypoblast ends near the umbilicus; but in some 

 mammals the hypoblastic diverticulum is prolonged to the inner surface 

 of the chorion. In man, therefore, the umbilical cord consists of 

 1, An outer covering of amnion ; 2, a core of modified mesoblast 

 derived from the mesoblast of the allantois and the wall of the yolk- 

 sac ; 3, the remains of the hypoblastic portion of the yolk-sac, and 

 4, the two allantoic arteries and the allantoic vein. 



In the early stages immediately after the separation of the amnion 

 from the chorion, the embryo and its amnion are attached to the 

 chorion by the allantois, and they are situated in a space which is 

 part of the original coelomic space between the somatic and splanchnic 

 mesoblast (figs. 635 to 637). This space is continuous with the coelum 



