666 MANUAL OF PHYSIOLOGY. 



cord, which runs from the apex of the bladder to the umbilicus, 

 and is known as the urachus. 



(4) The Chorion is the external covering of the ovum. At first 

 it consists simply of the zona pellucida or vitelline membrane, 

 and then it is called the primitive chorion. Later, however, it is 

 supplemented by the part of the somatopleure removed from the 

 embryo in the process of forming the amnion. This blends with 

 the primitive chorion and strengthens it, and while lying beneath 

 the zona pellucida, receives the name of the subzonal membrane. 

 The chorion at first is a smooth membrane, but villous processes 

 early grow out from it. These villi are chiefly developed at its 

 upper part, where they aid in the formation of the foetal placenta. 



The allantois, when it has spread over the chorion, becomes 

 blended with this membrane, and fills the villous processes with 

 the bloodvessels it contains. 



THE PLACENTA. 



The placenta is a most important organ to the mammalian em- 

 bryo. It conveys not only nourishment, but also oxygen from 

 the maternal blood to that of the fcetus. It is, of course, neces- 

 sary that the animals whose ova do not contain large stores of 

 food, should in some way provide the substances necessary for the 

 life of their embryo, and it is by means of the placenta that this 

 is brought about. The embryo of oviparous animals does not 

 require a placenta for its nutrition, since inside the egg is a large 

 store of highly nutritious albuminous and fatty materials ; the 

 shell is pervious to air, and the chick's blood can in the allantois 

 be oxidized by the air directly. A bird's egg contains in itself 

 all the necessaries which the placenta supplies, and when impreg- 

 nated only requires the heat of the mother's body to develop a 

 chick. 



While an ovum is descending the Fallopian tube, the mucous 

 membrane of the uterus becomes turgid, and, as before mentioned, 

 if the ovum be unimpregnated it is cast out of the body, part of 

 the substance of the lining membrane of the uterus is desqua- 

 mated and discharged with a fluid largely composed of blood. 

 This takes place approximately every four weeks, and hence is 



