914 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



its decidual wall, the segmented ovum continues to take nutriment from its 

 immediate environing cells. It has been suggested, but without much basis 

 of fact, that the uterine glands, which at this time are greatly dilated, may 

 furnish a nutritive secretion for the use of the embryo ; but, a priori, it would 

 seem more reasonable that, just as the ovum within the Graafian follicle 

 obtains its food from its surrounding stroma, so within the highly vascular 

 decidua it absorbs directly from the decidual tissue. But that this source 

 soon proves insufficient for the rapid growth is indicated by the early develop- 

 ment of the chorion with its villi and the embryonic vascular system. In 

 Reiehert's ovum, the earliest known human embryo, and believed to be 

 between twelve and thirteen days old, the villi are already well marked 

 over an equatorial zone. From this time onward throughout gestation the 

 chorion takes an important part in the embryonic nutrition, becoming, as we 

 have seen, an integral part of the placenta. The placenta is par excellence 

 the medium of nutritive communication between mother and child. 



Let us consider briefly the needs of the embryo. The fetal energies must 

 be directed almost wholly to the all-important functions of growth and prepa- 

 ration for the future independent existence. The organism requires, therefore, 

 an abundance of food containing all the chief kinds of food-stuffs. With the 

 alimentary canal in its embryonic and functionless state, this food, when it 

 reaches the embryo, must necessarily be already digested and ready for absorp- 

 tion by the cells. A supply of oxygen, not necessarily great in quantity, is 

 also needed. The fetal lungs are not ready for respiration, and the oxygen 

 must come to the blood by another channel than them. Carbonic acid must 

 be got rid of, and through other than pulmonary paths. Urea and its fore- 

 runners and other wastes, probably not in great quantity, must be excreted. 

 The fetal kidneys and the skin are probably never very active, as is made rea- 

 sonably certain by the late external opening of the male urethra, the late 

 development of the cutaneous glands, and the composition of the amniotic 

 fluid, into which they would naturally pour their secretions. Thus the paths 

 of income and outgo that are normal to the individual after birth are only 

 partially open during fetal life ; nevertheless, the processes of income and 

 outgo must be performed. The placenta, with its close relationship but non- 

 communication of maternal and fetal blood-vessels, has, therefore, been evolved 

 phylogenetically, and appears early in the course of ontogeny. To it is 

 brought on the part of the embryo and discharged into the villous capillaries 

 a mixed blood, comprising venous blood from the various capillary systems 

 of the body, and containing, therefore, the carbonic acid and other wastes 

 of venous blood, and a certain proportion of purified blood that has passed 

 directly by way of the ductus venosus, inferior vena cava, right auricle, fora- 

 men ovale, and the left side of the heart to the aorta and the umbilical arte- 

 ries. To it is brought on the part of the mother and discharged into the 

 sinuses pure arterial blood, laden with food and with oxygen. Through the 

 membrane intervening between maternal and fetal vessels there passes from 

 the fetus carbonic acid and other wastes, and from the mother food and oxy- 



