370 GENERATION. 



destined to form connections with, the mucous membrane of 

 the uterus, and constitutes the foetal portion of the placenta. 

 According to Dalton, this change begins at about the end 

 of the second month, and the placenta becomes distinctly 

 limited at about the end of the third month. 1 



It must be remembered that, as the changes progress 

 which result in the f ormation of the permanent chorion and 

 the limitation of the foetal portion of the placenta, the forma- 

 tion of the umbilical vesicle and the enlargement of the am- 

 nion are also going on. The amnion is gradually becoming 

 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 vesi- 

 cle, and the umbilical vesicle itself lies in the gelatinous mat- 

 ter between the two membranes. 



At about the beginning of the fifth month, then, the ovum 

 is constituted as follows : 



The foetus 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 por- 

 tion of the placenta ; the rest of the chorion is a simple mem- 

 brane, 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 vesi- 

 cle has become atrophied and has lost its vascularity ; the 

 hernia at.^he point of connection of the umbilical vesicle 

 with the intestine of the foetus has closed ; finally, the foetus 

 has undergone a considerable degree of development. 



It now remains for us to study the structure of the um- 

 bilical cord, the membranes formed from the mucous mem- 

 brane of the uterus, or the membranse deciduse, and the mode 

 of development and the structure of the placenta. 



Umbilical Cord. From the description we have given of 



1 D ALTON, Human Physiology, Philadelphia, 1871, pp. 632, 633. 



