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On the ground of these differences we divide it into two principal sections; the lower Placentals or Indecidua, and the higher Placentals or Deciduata. They are only loosely connected with the mucous coat of the uterus, so that the whole foetal membrane with its villi can be easily withdrawn from the uterine depressions like a hand from a glove.
Whether or not we were a fish in the Devonian seas, there is little doubt that we had gills, because we have the gill slits yet in our early foetal life, and it is quite certain that in some way we owe our backbones to the fishes.
We have already seen that this nucleated vesicle is itself a type of mature and independent being in the infusory animalcules, as well as the starting point of the foetal progress of every higher individual in creation, both animal and vegetable.
But it is one thing to say, Darwinically, that every detail observed in an animal's structure is of use to it, or has been of use to its ancestors; and quite another to affirm, teleologically, that every detail of an animal's structure has been created for its benefit. On the former hypothesis, for example, the teeth of the foetal 'Balaena' have a meaning; on the latter, none.
Since, however, men and women have descended from remote ancestors who, in the manner of aquatic creatures, exercised functions of sperm-extrusion and germ-extrusion that were exactly analogous in the two sexes, without any specialized female uterine organization, the early stages of human male and female foetal development still display the comparatively undifferentiated sexual organization of those remote ancestors, and during the first months of foetal life it is practically impossible to tell by the inspection of the genital regions whether the embryo would have developed into a man or into a woman.
But the calf has in its upper jaw some rudiments of teeth which never are developed, and never play the part of teeth at all. The whalebone whale, again, has horny "whalebone" plates in its mouth, and no teeth; but the young foetal whale, before it is born, has teeth in its jaws; they, however, are never used, and they never come to anything.
Such hypertrichosis is frequently degenerative in character, though still often associated with the sexual system. When, however, it is thus a degenerative character of sexual nature, having its origin in some abnormal foetal condition or later atrophy of the ovaries, it is no necessary indication of any aptitude for detumescence.
THE FOETAL HEART. In the fifth month there is a sign which, if detected, furnishes indubitable evidence of conception, and that is the sound of the child's heart. If the ear be placed on the abdomen, over the womb, the beating of the foetal heart can sometimes be heard quite plainly, and by the use of an instrument called the stethoscope, the sounds can be still more plainly heard.
The beating of the heart of the calf may then be distinctly heard, twice, or more than twice, as frequent as that of the mother; and each pulsation will betray the singular double beating of the foetal heart. This will also be accompanied by the audible rushing of the blood through the vessels of the placenta.
It is equally remarkable that analogous purposes are served in different animals by organs essentially different. Thus, the mammalia breathe by lungs; the fishes, by gills. These are not modifications of one organ, but distinct organs. In mammifers, the gills exist and act at an early stage of the foetal state, but afterwards go back and appear no more; while the lungs are developed.
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