808 



GENERATION. 



Fio. 296. Human embryon at the third week, showing 

 wlli covering the entire chorion (Haeckel). 



developed more rapidly than over the rest of the surface. Indeed, as the 

 ovum becomes larger and larger, the villi of the surface outside of this area 



become more and more scanty, 

 lose their vascularity and finally 

 disappear. That portion of the 

 allantois upon which the villi per- 

 sist and increase in length and in 

 the number of their branches is 

 destined to form connections with 

 the mucous membrane of the ute- 

 rus and constitutes the foetal por- 

 tion of the placenta. This change 

 begins at about the end of the sec- 

 ond month, and the placenta be- 

 comes distinctly limited at about 

 the end of the third month. 



It must be remembered that as 

 the changes go on which result in 

 the formation of the permanent 

 chorion and the limitation of the 

 foetal portion of the placenta, the 



formation of the umbilical vesicle and the enlargement of the amnion are 

 also progressing. The amnion is gradually distended by the increase in the 

 quantity of amniotic fluid. It reaches the internal surface of the chorion at 

 about the end of the fourth month, extends over the umbilical cord to form 

 its external covering, including the cord of the umbilical vesicle, and the 

 umbilical vesicle itself lies in the gelatinous matter between the two mem- 

 branes. 



At about the beginning of the fifth month the ovum is constituted as 

 follows : 



The fo3tus floats freely in the amniotic fluid, attached to the placenta by 

 the umbilical cord ; the chorion presents a highly vascular, thickened and 

 villous portion, the foetal portion of the placenta ; the rest of the chorion is 

 a simple membrane, without villi and 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 umbilical 

 vesicle with the intestine of the foetus has closed ; and finally the foetus has 

 undergone considerable development. 



Umbilical Cord. From the description given of the mode of develop- 

 ment 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 por- 

 tion 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 



