/Mm It ■ r>- 



1142 THE REPRODUCTIVE ORGANS 



a smooth and glistening appearance. The umbiUcal cord, containing 

 the blood-vessels which connect the fetus with the placenta, usually 

 enters near its center. It measures 1.0 to 2.5 cm. in diameter and 

 about 55 cm. in length. Its outer envelope consists of several layers 

 of epithelium which are directly continuous with the skin of the 

 fetus. Its connective tissue reticulum contains a mucoid substance, 

 known as the Whartonian jelly, which serves as a protection to its! 

 blood-vessels. The latter ramify extensively directly below the sur- 

 face of the amnion, so that they are already well subdivided before 

 they reach the chorion. 



The Nutrition of the Embryo. — In the earliest stage of its develop- 

 ment the embryo possesses no circulatory system, but derives its 

 nutritive material from the media surrounding it. Shortly afterward 

 there is developed the yolk-circulation which, however, does not 

 continue for any length of time, because the supply in this material 

 is very limited. For this reason, its place is taken during the third 

 week by the circulatory mechanism pf the chorion which is eventually 

 changed into the complete circulatory system of the placenta. The 

 latter becomes functional at the end of the second month of intra- 

 uterine life, so that the fresh blood henceforth leaves the placenta by 

 way of the umbilical vein, while the impure blood is returned to it 

 by way of the umbilical arteries. The fetal heart beats as a rule at 

 the rate of 120 to 140 in -a minute. It may be heard at first directly 

 over the symphysis pubis, and during the later months at a point 

 about midway between the umbilicus and the superior iliac spine, 

 according to the position of the fetus. ^ Actual movements of the 

 fetus are perceived at about the eighteenth or twentieth week. 



The oxygen requirement of the fetus is relatively small, because 

 the developing organism is protected against a loss of heat by the 

 mother. This implies that the heat produced in consequence of the 

 oxidations in its tissues is stored, giving rise to a temperature which 

 is usually somewhat higher than that of the mother. As far as the 

 actual transfer of oxygen is concerned, it has been proved that the 

 blood of the umbilical vein is lighter in color than that of the umbiUcal 

 arteries, and contains oxyhemoglobin. In this connection it might 

 also be mentioned that ether and chloroform, when administered to 

 the mother, are transferred to the fetal blood. 



The occurrence of a transfer of nutritive material is proved con- 

 clusively by the constant growth of the fetus. In this process the 

 chorionic epithelium plays a part analogous to that of the intestinal 

 wall, i.e., it subjects the nutritive substances to radical changes in 

 order to render them assimilable by the cells of the fetal tissues. 

 This is true of the albuminous material as well as of fats. Both are 

 first reduced into simpler compounds and are then rebuilt into tissue- 

 protein and body fat. In the case of the fat, Hofbauer^ has shown that 



1 First heard by Mayor of Geneva, in 1818.. 



2 Zeitschr. fur Geburtsh. und Gynec, Ixiv, 1909, 668. 



