64 



EMBRYOLOGY 





maternal blood, and give up to the latter its waste products. The blood, so purified , 

 is carried back to the fetus by the umbilical vein. It will thus be seen that the 

 placenta not only establishes a mechanical connection between the mother and the 

 fetus, but subserves for the latter the purposes of nutrition, respiration, and ex- 

 cretion. In favor of the view that the placenta possesses certain selective pow r ers 

 may be mentioned the fact that glucose is more plentiful in the maternal than in 

 the fetal blood. It is interesting to note also that the proportion of iron, and of 

 lime and potash, in the fetus is increased during the last months of pregnancy. 

 Further, there is evidence that the maternal leucocytes may migrate into the fetal 

 blood, since leucocytes are much more numerous in the blood of the umbilical vein 

 than in that of the umbilical arteries. 



The placenta is usually attached near the fundus uteri, and more frequently on 

 the posterior than on the anterior wall of the uterus. It may, however, occupy 

 a lower position and, in rare cases, its site is close to the orificium internum uteri, 

 which it may occlude, thus giving rise to the condition known as placenta previa. 



Stratum spongiosum 

 Limiting or boundary layer 

 Maternal vessels 



Placental septum 



Vittus 



Trophoblast 

 Amniort Umbilical arteries 

 Umbilical vein 



Chorion 

 Marginal sinus 



FIG. 39. Scheme of placental circulation. 



Separation of the Placenta. After the child is born, the placenta and membranes 

 are expelled from the uterus as the after-birth. The separation of the placenta from 

 the uterine wall takes place through the stratum spongiosum, and necessarily 

 causes rupture of the uterine vessels. The orifices of the torn vessels are, however, 

 closed by the firm contraction of the uterine muscular fibers, and thus postpartum 

 hemorrhage is controlled. The epithelial lining of the uterus is regenerated by the 

 proliferation and extension of the epithelium which lines the persistent portions 

 of the uterine glands in the unaltered layer of the decidua. 



The expelled placenta appears as a discoid mass which weighs about 450 gm. 

 and has a diameter of from 15 to 20 cm. Its average thickness is about 3 cm., 

 but this diminishes rapidly toward the circumference of the disk, which is continu- 

 ous with the membranes. Its uterine surface is divided by a series of fissures into 

 lobules or cotyledons, the fissures containing the remains of the septa which extended 

 between the maternal and fetal portions. Most of these septa end in irregular 

 or pointed processes; others, especially those near the edge of the placenta, pass 



