EMBRYOLOGY. 269 



positive of the previous existence of a fetus. They are characteristic 

 of the chorion, and are found under no other circumstances. 



At about the end of the second month the villosities begin to 

 atrophy and disappear from the surface of the chorion, with the ex- 

 ception of those situated at the points of entrance of the fetal 

 blood-vessels, which occupy about one third of its surface, where 

 they continue to grow longer, become more vascular, and ultimately 

 assist in the formation of the placenta ; the remaining two thirds 

 of the surface loses its villi and blood-vessels and becomes a 

 simple membrane. 



The umbilical cord connects the fetus with that portion of the 

 chorion which forms the fetal side of the placenta. It is a process 

 of the allantois, and contains two arteries and a vein, which have a 

 more or less spiral direction. It appears at the end of the first 

 month, and gradually increases in length until, at the end of gesta- 

 tion, it measures about twenty inches. The cord is also surrounded 

 by a process of the amnion. 



Development of the Decidual Membrane. The interior of the 

 uterus is lined by a thin, delicate mucous membrane, in which are 

 embedded immense numbers of tubules, terminating in blind ex- 

 tremities the uterine tubules. At each period of menstruation the 

 mucous membrane becomes thickened and vascular, which condition, 

 however, disappears after the usual menstrual discharge. When the 

 ovum becomes fecundated, the mucous membrane takes on an in- 

 creased growth, becomes more hypertrophied and vascular, sends up 

 little processes or elevations from its, surf ace, and constitutes the 

 decidua vera. 



As the ovum passes from the Fallopian tube into the interior of 

 the uterus, the primitive vitelline membrane, covered with villosities, 

 becomes entangled with the processes of the mucous membrane. A 

 portion of the decidua vera then grows up on all sides and incloses 

 the ovum, forming the decidua reflexa, while the villous processes 

 of the chorion insert themselves into the uterine tubules and in the 

 mucous membrane between them. 



As development advances, the decidua reflexa increases in size, 

 and at about the end of the fourth month comes in contact with the 

 decidua vera, with which it is ultimately fused. 



The Placenta. Of all the embryonic structures, the placenta is 

 the most important. It is formed in the third month, and then 



