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Updated: June 18, 2025
It is true that the lowest Placentals are not far removed from the Marsupials in cerebral development; but within the placental group we can trace an unbroken gradation of progressive development of the brain, rising gradually from this lowest stage up to the elaborate psychic organ of the apes and man.
For ten lunar months, not another ovum breaks through the covering of the ovary, and the uterus is left undisturbed. The placental secretion plays a most important rôle as brake upon the post-pituitary, the most active of the feminizing uterus-disturbing endocrines.
Perhaps the more common opinion is that they were evolved from a branch of the Insectivores, but the evidence is too slender to justify an opinion. It was an age when the primitive placental mammals were just beginning to diverge from each other, and had still many features in common.
Thus horses form a species, because the group of animals to which that name is applied is distinguished from all others in the world by the following constantly associated characters. They have 1. A vertebral column; 2. Mammæ; 3. A placental embryo; 4. Four legs; 5. A single well-developed toe in each foot provided with a hoof; 6. A bushy tail; and 7.
In proof of the first of these positions, we may cite the fact that, in the relative development of the limbs, the civilised man departs more widely from the general type of the placental mammalia than do the lower human races.
They have 1, A vertebral column; 2, Mammae; 3, A placental embryo; 4, Four legs; 5, A single well-developed toe in each foot provided with a hoof; 6, A bushy tail; and 7, Callosities on the inner sides of both the fore and the hind legs.
In the lower and smaller forms of placental Mammals the surface of the cerebral hemispheres is either smooth or evenly rounded, or exhibits a very few grooves, which are technically termed 'sulci, separating ridges or 'convolutions' of the substance of the brain; and the smaller species of all orders tend to a similar smoothness of brain.
We observe, for example, in the Triassic, Oolitic, and Cretaceous strata, not only an absence of placental mammalia, but the presence of innumerable reptiles, some of large size, terrestrial and aquatic, herbivorous and predaceous, fitted to perform the functions now discharged by the mammalia.
This is particularly important, because the retention of placental tissue affords opportunity for several unpleasant complications; and neglect in this regard accounts in part for the belief that miscarriage is certain to leave women irreparably broken in health.
One of them, the retention of placental tissue, has just been mentioned, but serious consequences resulting from it are almost inexcusable, for, although the placenta may separate less readily and be cast off less thoroughly after miscarriage, modern medical skill can successfully cope with such conditions. Another fruitful source of unfortunate after-effects is the imprudence of the patient.
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