FORMATION OF THE ALLANTOIS. 369 



portion of the aorta, and two veins. The two arteries per- 

 sist and form the two arteries of the umbilical cord, coming 

 from the internal iliac arteries of the foetus; and the two 

 veins are reduced to one, the umbilical vein, which returns 

 the blood from the placenta to the foetus. These vessels are 

 connected with the permanent vascular tufts of the chorion. 



Longet states, as the result of a study of the specimens in 

 the College of France, prepared by Coste, that the develop- 

 ment of the allantois cannot be well observed in human ova be- 

 fore the fifteenth or the twenty-fifth day. 1 "We have already 

 noted the formation of villosities, 

 first upon the vitelline membrane, 

 and next upon the external amniotic 

 membrane, and we have seen that 

 both of these membranes are tempo- 

 rary structures. As the vascular al-", 

 lantois encroaches upon the external 

 amniotic layer, the villosities become 

 vascular ; and, when the allantois be- 

 comes the permanent Chorion, it is Fecundated efflr, with allantots 



fully formed. a, umbilical 



marked by a multitude of compound JSr^^S^^ 

 villi over its entire surface, which *gw Philadelphia, ISTI, P . 

 give the ovum a shaggy appearance. 



It is difficult to say whether new villi appear upon the allan- 

 tois, or whether the villi of the amnion are penetrated by the 

 vessels of the allantois ; but it is certain that the true or per- 

 manent chorion presents upon its surface vascular villi. As 

 the ovum enlarges, over a certain .area surrounding the point 

 of attachment of the pedicle which connects it with the em- 

 bryon, the villi are developed more rapidly than over the 

 rest of the surface. Indeed, as the egg becomes larger and 

 larger, the villi of the surface outside of this area bteeome 

 more and more scanty, lose their vascularity, and finally dis- 

 appear. That portion upon which the villi persist and in- 

 crease in length and in the number of their branches is 



1 LONGET, Traite de physiologic, Paris, 1869, tome iii., p. 849. 



