Placental mammal

“ In the next stage, if we adopt the modern view that Marsupials are descended from Placental Mammals, the eggs would be retained for increasing periods in the uteri, and would be born in a well-developed condition, since lactation would demand active sucking effort on the part of the young. ”

“ The robustness of the new procedure is assessed using simulations that show the types of change that most concern biologists; speedups or slowdowns through time, transient rate changes, and rate changes that are do not follow a normal or transformed normal distribution, as well as stochastic variation. A revised estimate of the origin of placental mammals is as young as 84 mya, which is much more recent than current estimates using molecular data. ”

“ The root of living placental mammals has been reported to be in the range of 100–140 mya [5] , [8] – [9] even with application of rate-adjustment techniques [5] , [9] . The molecular consensus of an old root is becoming strong enough that it may become dogma without a close examination of the assumptions being made in such analyses. The true age of the orders and superorders has important implications for determining the overall paleoecology and biogeography of placental mammals during a period that included the breakup of continents and the extinction of terrestrial dinosaurs. ”

“ Amongst the existing Mammals provided with a true placenta, the most primitive type is probably retained by those forms in which the placental part of the chorion is confined to a comparatively restricted area on the dorsal side of the embryo; while the false chorion is formed by the [Pg 241] vascular yolk-sack fusing with the remainder of the subzonal membrane. In all the existing forms with this arrangement of fœtal membranes, the placenta is deciduate. ”

“ The third and last sub-class of mammals comprises the Placental animals, or Placentals (Monodelphia, or Placentalia) . It is by far the most important, comprehensive, and most perfect of the three sub-classes; for the class includes all the known mammalia, with the exception of Marsupials and Beaked animals. Man also belongs to this sub-class, and has developed out of its lower members.
Placental animals, as their name indicates, are distinguished from all other mammals, more especially by the formation of a so called placenta. ”

“ In the higher mammals the womb becomes more and more developed, opening into a single vagina in the middle line of the abdomen, between the two ovaries, and constituting a highly specialized organ which allows the mother to preserve the young for a long time in her belly. In most mammals the uterus has two elongated diverticula, each of which may contain a successive series of embryos. In man it forms a single cavity and normally contains a single embryo, occasionally two or more. These facts show that the role of the female mammal in reproduction is more important than that of the male. ”

“ THE PLACENTAL STAGE.—Although the third stage is chiefly concerned with the separation and the delivery of the after-birth, on which account it is known as the placental period, the description of other no less remarkable events belongs here. Even after the infant is born the umbilical cord extends from its navel to the placenta, just as it has done throughout pregnancy. Among larger mammals separation of the new-born from the mother is brought about in one of two ways; sometimes the activity of the young breaks the navel-string, though more frequently the mother bites it in two. ”

“ It will be remembered that in Man, before the true placenta becomes developed, there is a condition with simple vascular villi scattered over the chorion. It seems very probable that this is a repetition of the condition of the placenta of the ancestors of the Primates which has probably been more or less retained by the Lemurs. It was mentioned above that the resemblance between the metadiscoidal placenta of Man and that of the Cheiroptera, Insectivora and Rodentia is rather physiological than morphological. ”

“ In a broad sense, the period of gestation for each variety of mammal is determined by the time required for embryonic development to reach the point where the young may live independently of the mother. This point is reached more quickly with small animals than with large. The mouse, for example, generally brings forth its young in three weeks, whereas the pregnancy of the elephant lasts two years. In human beings, counting from the time of conception to the time of delivery, pregnancy continues approximately 273 days. ”

“ A new-born kitten is a much feebler, less developed being than a new-born calf. An embryonic appendage, the allantois, used in reptiles and birds for respiration, has here been turned to another purpose. It lays itself against the walls of the uterus, uterine projections interlock with those which it puts forth, and the blood of the mother circulates through a host of capillaries separated from those of the blood system of the embryo only by the thinnest membrane. This is the placenta, developed, in part from the allantois of the embryo, in part from the uterus of the mother. ”

“ For some embryos have a more perfect structure provided for them, such as that fleshy substance mentioned above, which in some is altogether wanting. In some, also, the placenta is observed to be thicker, larger, and more loaded with blood; whilst in others it is more spongy and white, like the thymus or pancreas. But there is not more variety found in the placenta than in the mamma or viscera generally: for instance, the liver in some animals is red and filled with blood, in others, as is the case with fishes and some cachectic persons in the human species, it is of much paler hue. ”

“ It becomes a most important matter to ascertain the limits of the placental power, and by observation upon human beings and experiment upon the lower animals this matter has been very thoroughly elucidated of late years. There are many kinds of poison, and many varieties of those living poisons that we call microbes, which the placenta does not allow to pass through from the mother's blood-vessels into those of the child, and which are unable, fortunately for the child, to break down the placental resistance. ”

“ The Lemurs in spite of their affinities with the Primates and Insectivora have, as has been shewn by Milne Edwards and Turner, an apparently very different form of placenta. There is only one embryo, which occupies the body and one of the cornua of the uterus. The yolk-sack disappears early, and the allantois (Turner) bulges out into a right and left lobe, which meet above the back of the embryo. The cavity of the allantois persists, and the mesoblast of the outer wall fuses with the subzonal membrane (the hypoblastic epithelium remaining distinct) to give rise to the chorion. ”

“ If the union were vascular, the mother's blood would circulate in the foetal body, and the impulses of the maternal heart might prove too strong for the delicate organism of the embryo. Besides, the separation of the placenta from the uterus might prove fatal to both parent and offspring. The placenta is only a temporary organ, and when its functions are no longer required, it is easily and safely removed. ”

“ He adds the Number, Shape, and Use of these Placentas; and first observes that those that are Kernel-bearing Animals, or chewing the Cud, have many, and those that are Cake-bearing, have for the most part, one Cake for each Fœtus; but a woman commonly but one, though she happen to have many Embryo's.
He annexes a particular description of the Placenta of a Woman, as the most considerable, and teaches, how it may be most conveniently severed from the Vessels, to render them conspicuous, which are a numerous off-spring of Arteries, Veins, and Fibres ”

“ At childbirth, the placenta, which is considered as sacred, is carefully put away from the reach of the dogs as soon as thrown off from the uterus, and the female is up and following her usual avocations a very few hours after the accouchement. ”

“ The comparative growth of the fœtus infinitely surpasses that of its whole future existence. But the exact age of uterine embryos, and of abortives, is, in some degree, conjectural. Before the seventeenth day after conception, its rudiments are not even visible to the naked eye: the glary ovum has then slidden down from the ovarium, through one of the lateral tubes into the womb. The size of an abortion, comprehending the fœtus, its membranes, waters, and placenta, at the end of six weeks, does not exceed in size that of a pigeon’s or hen’s, and in three months, a goose’s egg. ”

“ When the segmenting egg attaches itself to the uterine wall it does not only produce the embryo proper, but it gives also rise to certain cells which attach themselves to the uterine wall of the mother, and are mainly concerned in transferring food from the mother to the embryo, thus forming the embryonal placenta, the outer layer of which is called the chorionic. These chorionic cells, which are therefore of embryonic origin, migrate already under normal conditions deeply into the uterine wall ”

“ If such be the case, can it have the power of retaining the germ in the womb, when on the most minute examination of the young, I could not detect, by cicatrice or line of abrasion on any part of the abdomen, that they ever possessed umbilical vessels, or had been in any way nourished by a placenta? Let us take into consideration the small size of the animal found in the pouch, its utter helplessness, its slight power of motion, and its firm attachment to the nipple. The more it is in the embryonic state the firmer is its attachment to the mother ”

“ We find only in the anthropoid apes—the gibbon and orang of Asia and the chimpanzee and gorilla of Africa—the peculiar and elaborate formation of the placenta that characterises man (Fig. 198) . In this case there is at an early stage an intimate blending of the chorion of the embryo and the part of the mucous lining of the womb to which it attaches. The villi of the chorion with the blood-vessels they contain grow so completely into the tissue of the uterus, which is rich in blood, that it becomes impossible to separate them, and they form together a sort of cake. ”

“ The first time was in a placenta from the garbage can. We had been culturing polio viruses in human placentas (very interesting personalities viruses are, too) and I'd been sent a whole placenta more than I needed. What can a mother tell a placenta which has been doing its work and is still in excellent shape, just like that civilization in the Phoenix Nebula some two and a half millennia ago? Does she say, 'There's nothing more for you in time or space; the baby is born, I abandon you to utter nothingness'? Very rarely. And even then she doesn't mean it. But the life does go out of the cells. ”

“ When this connection (called the placenta) between embryo and mother through the egg-shell became more perfect, not only oxygen but food-material was obtained from the blood-vessels of the mother; and, in consequence, it became unnecessary for the eggs to be provided with a large supply of food-yolk. Among existing marsupial animals, which, on the whole, represent a lower type of mammalian structure than ordinary mammals, there is more food-yolk than in ordinary mammals, and less food-yolk than in the two egg-laying mammals. ”

“ The utter loss of direct influence by the female bird upon its offspring from the time the egg has left her, and the marked effect, originally, of the male; the independence in body, in movement, and in life, of young marsupial mammals, almost from the very moment of their conception, identical analogically with the intra-uterine state of other embryos,—nourishment by teat merely replacing that by placenta at an earlier period ”

“ Mere arrest of development of a part may arise from accidental disease of the embryo; hence vital organs are left out, or portions of organs, like the dividing walls of the heart, are omitted. Sometimes an older fetus is inclosed in the body of another, each having started independently from a separate ovum, but the one having become embedded in the semifluid mass of the other and having developed there simultaneously with it, but not so largely nor perfectly. ”

“ But in the Placentals, where gestation lasts a long time, and the embryo reaches its full development under the protection of its enveloping membranes, there has to be a new mechanism for the direct supply of a large quantity of food, and this is admirably met by the formation of the placenta. ”

“ Now please remember that it is right that the placenta should grow fast to the womb; that is, to its inner surface. It is always grown fast, and should be; but it is possible for it to adhere more firmly in some cases than in others; and in some cases, too, the uterus seems to be so weak that it has not power sufficient to expel it.
In cases when the womb is very active after the birth of the child, the placenta may be expelled very quickly. ”

“ Mayrhofer, however, opposes this assumption by the following deductions (“The Determination of Sex in the Human Species”) ; “Fœtuses enclosed within a single chorion always possess a common placenta, in which the blood-channels from both umbilical cords frequently, in the case of twins perhaps invariably, communicate. It might therefore be supposed that the sex-identity of embryos enclosed within a single chorion is due to the intermixture of their blood in the placenta. ”

“ Its shreds when expelled are of diagnostic importance, and in early abortions its massive thick tissues, changed by disease, often cause greater trouble than the ovum itself, forming, alone or with the membranes proper of the ovum, what is so commonly but erroneously called the placenta in abortion. ”