THE DEVELOPMENT OF THE EMBRYO 1141 



rounded bag of water and enables the contracting uterus to bring an equal pressure 

 to bear upon the entire circumference of the cervix, thereby preventing tears. 



The Function of the Placenta.— At about the fourth month, this 

 organ consists chiefly of villi of the chorion which contain connective 

 tissue cells and numerous blood-vessels. The cells of the tips of these 

 projections proliferate very actively and finally invade the decidua. 

 The spaces between these projections, as well as those separating the 

 individual villi, are filled with maternal blood, which gains entrance 

 to them through free openings in the maternal blood-vessels. It will 

 be seen, therefore, that the maternal blood remains separated from 

 that of the fetus by the double layer of epithelium of the chorion and 

 the stroma and walls of the blood-vessels of the villi. Consequently, 

 the placenta really represents a mass of maternal blood which has 

 been temporarily diverted into the spaces between the chorionic 

 membrane and the decidua basalis of the uterus. Into this blood 

 project the capillary coils of the different villi, without, however, 

 establishing a direct connection between these two types of blood. 



Regarding the manner in which the constituents of one are inter- 

 changed for those of the other, nothing further can be said than has 

 already been mentioned when discussing the causes underlying the 

 formation of any secretion. Diffusion and osmosis are augmented 

 by a vital activity of the lining cells of the chorionic villi. Evidently, 

 these cells play the part of a gland. The oxygen and nutritive par- 

 ticles are made to pass into the umbilical vein of the fetus, whereas 

 the waste products of the latter are directed from the capillaries of 

 its two umbilical arteries into the maternal venous system. Fat 

 globules have been observed to traverse the chorionic lining cells 

 and, besides, these cells serve as storehouses of glycogen, from which 

 the fetus may derive extra amounts of sugar whenever in need of 

 them. Attention should also be called to the fact that considerable 

 quantities of glycogen are deposited in the uterine mucosa some time 

 before each menstrual period, a provision of Nature which evidently 

 is made in anticipation of the arrival of a fertilized ovum. Thus, it 

 will be seen that the placenta really represents a combination of organs, 

 because it serves jointly as the respiratory, digestive and excretory 

 mechanisms of the developing young. By analogy it may then be 

 concluded that the fetus really occupies the position of a tissue, its 

 nutrition being cared for as if it were an actual part of the maternal 

 body. Practically any substance contained in the mother's blood 

 may find its way into the fetus. 



Within 10 to 20 minutes after the expulsion of the fetus, the 

 placenta is cast off from the uterus, forming what is known as the 

 after-birth. The latter appears as a flat, rounded plate, weighing 500 

 to 600 grams, and measuring 15 to 18 cm. in diameter and 2 to 3 cm. 

 in thickness. Its outer or maternal surface is rough and presents 

 numerous irregular depressions, or cotyledons, while its inner or fetal 

 surface is covered by amniotic membrane and possesses, therefore. 



