Fig.196. Diagrammatic frontal section of the pregnant human womb. Fig. 196—Diagrammatic frontal section of the pregnant human womb. (From _Longet._) The embryo hangs by the umbilical cord, which encloses the pedicle of the allantois (_al_). _ nb_ umbilical vessel, _am_ amnion, _ch_ chorion, _ ds_ decidua serotina, _dv_ decidua vera, _dr_ decidua reflexa, _z_ villi of the placenta, _c_ cervix uteri, _ u_ uterus.)
The pedicle of the allantois, which connects the embryo with the placenta and conducts the strong umbilical vessels from the former to the latter, is covered by the amnion, and, with this amniotic sheath and the pedicle of the yelk-sac, forms what is called the _umbilical cord_ (Fig. 196 _al_). As the large and blood-filled vascular network of the fœtal allantois attaches itself closely to the mucous lining of the maternal womb, and the partition between the blood-vessels of mother and child becomes much thinner, we get that remarkable nutritive apparatus of the fœtal body which is characteristic of the placentalia (or choriata). We shall return afterwards to the closer consideration of this (cf. Chapter XXIII).
In the various orders of mammals the placenta undergoes many modifications, and these are in part of great evolutionary importance and useful in classification. There is only one of these that need be specially mentioned—the important fact, established by Selenka in 1890, that the distinctive human placentation is confined to the anthropoids. In this most advanced group of the mammals the allantois is very small, soon loses its cavity, and then, in common with the amnion, undergoes certain peculiar changes. The umbilical cord develops in this case from what is called the “ventral pedicle.” Until very recently this was regarded as a structure peculiar to man. We now know from Selenka that the much-discussed ventral pedicle is merely the pedicle of the allantois, combined with the pedicle of the amnion and the rudimentary pedicle of the yelk-sac. It has just the same structure in the orang and gibbon (Fig. 197) and very probably in the chimpanzee and gorilla, as in man; it is, therefore, not a _disproof,_ but a striking fresh proof, of the blood-relationship of man and the anthropoid apes.