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The net result of this scheme was that not only did the head and upper 
extremities receive a better quality of blood, but a right venous and a 
left arterial heart was maintained and a function was suggested for the 
Eustachian valve in the right auricle. Unfortunately this doctrine has 
been antagonized since 18385 with little effect on the described circulation 
in the mammalian fetus, and with no consideration of its evident defects 
in the latest text-book (8) on chick embryology. At the last meeting of the 
Academy I labeled the scheme ‘‘morphologically inaccurate, developmentally 
unnecessary and physically impossible.” The second theory (Wolff) was 
based on excellent anatomical observation but does not fulfil the physical 
requirements of the proposition. The third theory (Harvey), a mixing of 
the blood in the right auricle, was quite definitely demonstrated to occur 
in the living fetal pig. I found by injection experiments that the blood 
passing into the heart from the right precaval and the postcaval veins 
found its way into both ventricles. Interpreted in a physiological man- 
ner, the result is that all the arteries in the mammalian embryo contain 
a mixed blood. The point raised, while of no practical importance in itself, 
is interesting because it was first suggested by Harvey in 1628; because 
it may lead to a more perfect understanding of the anatomical changes 
from the fetal to the adult circulation; and lastly because of its morpho- 
logical significance. It is the latter point that I would bring out in greater 
detail. 
It is well known that the double circulation is found only in the warm- 
blooded adult vertebrates (bird and mammal); animals in other words, 
where the body temperature demands a greater degree of oxygenation and 
in which the oxygenation is entirely confined to the lungs. In the lower 
vertebrates this condition does not obtain, reptiles excepted. The amphibian 
has other means of obtaining oxygen than through the lungs, and the fish, 
other paths than through the gills. The relatively low body temperature 
does not necessitate so rich a content of oxygen in the blood. If we ex- 
amine this statement closely we see that the embryos of mammal and bird 
resemble the reptile and amphibian; they do not possess a distinet four- 
chambered heart, and while in the latter the element of warmth does not 
enter, in the former all of the warmth, practically speaking, is supplied 
by the maternal body through internal or external incubation. The meta- 
bolic processes of the mammal and bird are therefore insufficient to main- 
tain the essential body temperature. 
If we examine the phylogenetic relation of the mammal and bird we 
