238 MANUAL OF ANIMAL BIOLOGY 



from the placenta is the embryonic umbilical cord which is di- 

 rectly attached to the body of the embryo. (W. f. 134, b, h, i.) 



The placenta is the organ by means of which the essential inter- 

 change of materials between the embryo and the mother takes 

 place. Certain large arteries of the mother bring maternal blood, 

 carrying nutritive materials and oxygen, to the placenta, where it 

 flows into large open sinuses. Here the maternal blood bathes 

 the finger-like projections, or villi, formed from fetal tissue. The 

 fetal villi contain the endings of the fetal blood vessels which run 

 through the umbilical cord and connect with the vascular system 

 of the embryo. One of the fetal vessels (umbilical artery) 

 brings blood to the placenta from the embryo. This blood con- 

 tains nitrogenous wastes and carbon dioxide given off by the 

 embryo. During the passage through the capillary network in the 

 placenta these wastes are given off through the walls of the villi 

 and pass into the maternal blood. Synchronously, nutritive mate- 

 rials and oxygen are received from the maternal blood. The em- 

 bryonic blood, now freed from the wastes and containing the sub- 

 stances necessary for the embryo, passes into the umbilical veins, 

 which carry it away from the placenta and back to the embryo 

 through the umbilical cord. It should be emphasized that the vas- 

 cular systems of the mother and embryo are not directly connected 

 in the placenta or elsewhere. All interchange of materials between 

 the mother and the developing embryo in the uterus must, there- 

 fore, take place by diffusion through the embryonic tissues in the 

 placenta. 



The period of development within the uterus, which is termed 

 gestation, varies greatly in the different Mammals. Thus in the 

 Mouse the normal period of gestation is 20 days, while in Man it 

 is approximately 280 days. When the mammalian embryo has 

 completed the necessary development within the uterus, birth 

 occurs. This is largely brought about by powerful rhythmic con- 

 tractions of the involuntary muscle tissue present in the walls of 

 the uterus. The final result of the contractions is the complete 

 separation of the embryo and the fetal tissues from the maternal 

 tissues, and the passage of the embryo to the exterior, where an 

 independent existence is begun. 



REFERENCES 



Consult the list given for the previous chapter (p. 221). 



