158 HUMAN PHYSIOLOGY. 



The Chorion, the external investment of the embryo, is formed by a 

 fusion of the original vitelline membrane, the external layer of the amnion, 

 and the allantois. The external surface now becomes covered with villous 

 processes, which increase in number and size by the continual budding 

 and growth of club-shaped processes from the main stem, and give to the 

 chorion a shaggy appearance. They consist of a homogeneous granular 

 matter, and are penetrated by branches of the blood vessels derived from 

 the aorta. 



The presence of villous processes in the uterine cavity is proof positive 

 of the previous existence of a foetus. 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 exception of those 

 situated at the points of entrance of the foetal 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 foetus with that portion of the 

 chorion which forms the foetal 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 gestation, 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 im- 

 bedded immense numbers of tubules, terminating in blind extremities, 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 increased growth, becomes more hyper- 

 trophied and vascular, sends up little processes, or elevations from its sur- 

 face, 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 encloses the ovum, forming 

 the decidua reflexa, while the viilous processes of the chorion insert them- 

 selves into the uterine tubules, and in the mucous membrane between 

 them. 



