The authors who regard this embryonic form as the primary gastrula of the placental conceive the outer layer as the ectoderm and the inner as the entoderm.
We find a decidua of this kind in most of the higher placentals; but it is only in man and the anthropoid apes that it divides into three parts--the outer, inner, and placental decidua.
The placental or spongy decidua (placentalis or serotina, Figure 1.
The placental decidua serotina is that part of the preceding which is completely transformed by the ingrowth of the chorion-villi, and is used for constructing the placenta.
However, opinions still differ considerably as to the real nature of this "provisional gastrula" of the placental and its relation to the blastula into which it is converted.
Gastrula of theplacental mammal (epigastrula from the rabbit), longitudinal section through the axis.
The cenogenetic gastrulation of the placental has been greatly modified by secondary adaptation in the various groups of this most advanced and youngest sub-class of the mammals.
Each of two females, captured on January 13, carried fiveplacental scars; one of the females was lactating.
In proof of the first of these positions, we may cite the fact that, in the relative development of the limbs, the civilized man departs more widely from the general type of the placentalmammalia than do the lower human races.
While the terminations of the placentalarteries and veins are spread in fine reticulation on the sides of these cells.
Placental vessels inserted into the arteries of the mother.
For example, a female of category 2, taken on August 12, and probably in postjuvenal pelage, had placental scars.
No female was recorded to have contained embryos, but two had enlarged uteri or placental scars.
These animals areplacental mammals, which, probably in secondary times, arose from terrestrial Mammalia, by adaptation to an aquatic life.
In all these cases the area of the placental villi appears to have increased so as to cover nearly the whole subzonal membrane, without the villi increasing to any great extent in complexity.
Comparing the butterflies (Rhopalocera) to the placental mammals, we may look upon the different families as similar to the orders of the mammalia.
Even if we entirely ignore it, all that we have learned from the zoological facts of comparative anatomy and ontogeny as to the placental character of man remains untouched.
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.
The zoology of Australia and Tasmania presents a very conspicuous point of difference from that of other regions of the globe, in the prevalence of non-placental mammalia.
We pass on to the other curious order of non-placental mammals, that of the Monotremata, so called from the structure of their organs of evacuation with a single orifice, as in birds.
For instance, the quantitative and qualitative development of the brain of the placental mammals during the Tertiary period is very remarkable.
Those conditions are certainly found in the higher vertebrates, especially in the placental mammals, the class from which man has sprung.
The fetus has an independent circulatory system of its own, and yet, at the same time, from the moment of the placental connection until the moment of childbirth, all its nourishment is derived from its mother.
The purpose of the umbillical cord is to contain and support the umbillical arteries and veins through which the fetus obtains nourishment from the placental substance, and through which the return blood flows.
In the class of mammals the steps are not difficult to conceive which led from the ancient Monotremata to the ancient Marsupials; and from these to the early progenitors of the placental mammals.
After syphilis the cause of premature labor next in frequency is nephritis with placental hemorrhages and infarcts.
When the rupture or abortion does not tear the placental site the fetus may develop in the abdominal cavity.
When the ectopic ovum begins to develop in the Fallopian tube the placental villi erode the tubal wall and the blood-vessels.
When the contracture is sufficient to let the fetus just engage, pressure may interfere with the placental circulation and kill the child.
Again, the placental site is raised, it feels rough, and the furrows in it lead one to think part of the placenta is still adherent, whereas all has been removed.
Infection of the child in utero, however, is extremely rare even by the placental way.
Such diseases are caused by a lack of mineral salts, which normally are carried to the fetus by the placental blood during the last two months of gestation.
From these spaces there pass off oblique utero-placental veins, which pierce the serotina, and form a system of large venous sinuses in the adjoining uterine wall (fig.
In the later periods of pregnancy the placental area becomes, however, in most forms much more restricted than the area of contact between the uterus and chorion.
The allantoic cavity persists (Owen), but the allantois only covers the placental area of the chorion.
With the full growth of the allantois there is formed a broad placental zone, with numerous branched villi, fitting into corresponding pits which become developed in the uterine walls.
In the Tenrec (Centetes) the yolk-sack and non-placental part of the chorion are described by Rolleston as being absent, but it seems not impossible that this may have been owing to the bad state of preservation of the specimen.
At the edge of the placentalzone there is a very small portion of the uterine mucous membrane reflected over the non-placental part of the chorion, which forms a small reflexa analogous with the reflexa in Man.
It probably originated in connection with the loss of food-yolk which took place on the establishment of a placental nutrition for the foetus.
The placental region of the chorion may in such cases be distinguished as the true chorion, from the remaining part which will be called the false chorion.
The features of the development of the placental Mammalia receive their most satisfactory explanation on the hypothesis that their ancestors were provided with a large-yolked ovum like that of the Sauropsida.
The allantois is adherent to the whole chorion, the non-placental parts of which are vascular.
If there is any probability of life, the physician should give them baptism before the placental circulation is stopped.
These four kinds of omphalosites are either dead when born, or they die as soon as the placental circulation is cut off.
We have also the remarkable example of Australia, with a much richer flora than that of the early Mesozoic, yet inhabited only by non-placental mammals, like those of the Mesozoic.
The placental mammals, the highest group of the animal kingdom, are not known till the beginning of the Eocene Tertiary.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "placental" in a variety of sentences. We hope that you will now be able to make sentences using this word.
