xxv] 



EUTHERIA 



665 



covering the allantois and the lining of the uterus degenerate, 

 allowing the blood-vessels of mother and embryo to come into close 

 contact. The placenta becomes gorged with blood driven into it 

 by the heart of the developing embryo, and at the same time the 

 uterus becomes congested and loses its epithelium, so that the blood 

 of the mother and that of the young approach very closely to 

 each other. They are separated only by the thin outer wall of 

 the placenta, so that nourishment diffuses from one to the other, 

 and the blood of the embryo is oxygenated and its carbon dioxide 

 removed by the maternal blood. So close is the connection, that 

 when the embryo is born and passes out of the uterus, carrying 



FIG. 332 



Diagrams to illustrate the arrangement of the female genital ducts 

 in an Eutherian Mammal. A. Babbit. B. Man. 



1. Ovary. 



2. Oviducal funnel. 5. Fallopian tube. 

 7. Vagina. 8. Urino-genital sinus. 



6. Uterus. 



with it the placenta, the latter in most cases tears open the vessels 

 in the wall of the uterus and the mother loses a considerable quan- 

 tity of blood. The lowest parts of the two oviducts are completely 

 joined and pass into a single passage, the vagina, while the middle 

 portions, or uteri, are sometimes quite separate as in the Rabbit 

 (A, Fig. 332), sometimes partly united as in the Cat, rarely com- 

 pletely joined as in Monkeys and Man (B, Fig. 332). In one or two 

 Metatheria, as already mentioned, a placenta such as has been 

 described has been recently discovered, but it is of very small 

 extent, and does not persist until birth. These facts lead us to 

 believe that Metatheria are degenerate descendants of early Eutheria, 



