While these two openings are free, which is constantly the case in the fœtus, the two systems evidently make but one, as I have said; whence it clearly follows that the blood that circulates there must be entirely of the same nature, that there cannot be two kinds in the fœtus, as there always is in the adult. This is, in fact, a remarkable difference between the two ages. 1st. I have many times dissected small Guinea pigs in the womb of the mother; their vessels have uniformly contained the same fluid, which is blackish, like the venous blood of the adult. This experiment is easy. The abdomen of the mother being divided, we successively open each of the separate sacs that the womb has for each fœtus. When one of these sacs is laid open, we cut the membranes, then the abdomen of the small animal, leaving the umbilical vessels untouched. The transparency of the parts easily allows us to see the uniformity of the colour of the blood of the vena cava and the aorta. The same remark applies to the superior parts. The carotid and jugular pour out the same blood when they are opened. 2d. I have three times made the same observation upon the fœtus of a dog. 3d. We know that the blood of the umbilical arteries is always black; all accoucheurs have remarked this. 4th. The change of the black blood to red arises from the contact of air in the lungs; the fœtus not breathing, cannot then have this kind of blood. 5th. I have dissected many fœtuses that have died in the womb of the mother; the blood of the veins and the arteries has appeared to me to be uniformly the same. It is true that this is not a very conclusive proof, since the mere standing of the red blood in the vessels, for a considerable length of time, is sufficient to make it black, as Hunter has observed. The preceding facts are sufficient to establish incontestibly the uniformity of the blood in the two systems of the fœtus; an uniformity that exists at least in external appearance, if it is not real in its intimate composition. It is for the chemists to elucidate this point.