The outer one corresponds with the membrane usually regarded in other Vertebrates as a chorion or product of the follicular epithelium, but, by tracing it back to its first origin, I have been led to reject this view of its nature.
In a general way it may be laid down that variations in any direction which tended to increase the absorbing capacities of the chorion would be advantageous.
The chorion is thus the root or primary cyst of the vascular system.
Originally the whole chorion is replete all around with vessels; but as the process of oxydation occurs most powerfully upon the fundus uteri, so are the vessels developed most abundantly in that very situation, and form the placenta.
The Mussels are embryos, in whom the liver originates, and whose chorion acquires a placenta.
Although the vascular supply is cut off from the chorion laeve, the processes on its surface do not completely abort.
It may be noted that the condition of the chorion at this stage is very similar to that of the normal diffused type of placenta, described in the sequel.
The blood-vessels of the chorion are derived from the usual allantoic arteries and vein.
While the ovum is still within the chorion five peculiar processes of epiblast grow out; four of which usually lie in the same sectional plane of the embryo.
A and B, a true chorion has already become established.
From the whole surface of the chorion there project branched vascular processes, covered by an epithelium.
The chorion frondosum, together with the decidua serotina, gives rise to the placenta.
As has already been mentioned, the placenta is formed of the decidua serotina and the foetal villi of the chorion frondosum.
They are generally distributed, though absent at the two ends of the chorion and opposite the os uteri.
This part of the chorion does not develop vascular villi.
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.
We thus get a differentiation between the smooth chorion (chorion laeve, Figure 2.
Here again the whole surface of the chorionis thickly covered with the villi in the beginning.
The latter is made up of the mucous coat of the uterus and its blood-vessels, the former of the tufted chorion and the umbilical vessels of the embryo (cf.
We have already considered the formation and significance of this remarkable embryonic organ when we traced the development of the chorion and the allantois in the human embryo (Chapter 1.
In these Indecidua the villi are distributed over the whole surface of the chorion (or its greater part) either singly or in groups.
Still more characteristic of the Deciduates is the peculiar and very intimate connection between the chorion frondosum and the corresponding part of the mucous coat of the womb, which we must regard as a real coalescence of the two.
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 yelk-sac is rather smaller, the amnion and chorion larger.
The chorion forms a spacious vesicle, to the inner wall of which the small foetus (to the right above) is attached by a short umbilical cord.
The villi of the chorion grow larger and more numerous.
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.
When the allantois has attached itself to the inner wall of the chorion and been converted into the placenta, its blood-vessels alone effect the nourishment of the embryo.
When we open thechorion of a human embryo of three weeks, we find on the ventral side of the foetus a large round sac, filled with fluid.
Within the outer chorion the tiny curved foetus lies on the large embryonic vesicle, to the left above.
As a large quantity of fluid gathers inside it, the chorionexpands more and more, so that the embryo only occupies a small part of the space within the vesicle.
From this stage the envelope is called the external membrane, the primarychorion or prochorion (a).
But in the third sub-class of the mammals the serolemma forms, by invagination at its outer surface, a number of hollow tufts or villi, from which it takes the name of the chorion or mallochorion.
The chorion is the outer envelope or membrane of the fetus, completely inclosing the fetus with its other membranes.
In the uterus the fetus is surrounded by several membranes which are known as the amnion or inner layer, the allantois or central layer, and the chorion or outer layer.
The fetal portion (D) consists of a mass of very minute hairlike processes on the chorion (E), which fit into corresponding depressions or pits of the maternal portion.
Covering the external face of the amnion and lining the inner face of the chorion is a double membrane, the allantois.
The placenta is a highly vascular structure spread out or scattered over the surface of the chorion and the mucous membrane of the uterus, that attaches the foetus and its envelopes to the womb (Fig.
They are enveloped in a thick chorion formed of several layers; and (during the later stages of development at any rate) there is a membrane within the chorion which exhibits clear indications of cell outlines[210].
The changes so far described are included within what Kowalevsky calls his first embryonic period; at its close the parts contained within the chorionhave the arrangement shewn in fig.
Later embryo, just before the splitting of the chorion from the side.
There is always a chorion formed as a cuticular deposit of the follicle cells, which is frequently sculptured, finely perforated, etc.
With the further growth of the embryo the chorion becomes split and cast off, the embryo being left enclosed within the inner membrane.
The eggs are laid in April, May, and June, and are enclosed in a kind of chorion with calcareous protuberances.
The original chorion appears to serve as cuticle, and is perforated by the cilia.
It is in many cases doubtful whether a particular membrane is a chorion or a vitelline membrane.
The chorion is covered by numerous rounded prominences, and occasionally exhibits a pattern corresponding with the areas of the cells which formed it.
The ovum is enclosed in a delicate vitelline membrane, enveloped in its turn by a chorion secreted by the walls of the oviduct.
After leaving the chorion the embryo Julus remains connected with it by a structureless membrane which is probably the outer of the two cuticular membranes.
The chorionis often ornamented with various processes, etc.
The eggs are oval, the chorion being prolonged into a single filamentary process or “holdfast.
This chorion is very fragile; it frequently gives way under slight pressure from the thin plate of glass which covers the object.
The constitution of this chorion is perfectly identical in the eggs both before and after impregnation.
The chorion is the external membrane; it is thick and spongy, sprinkled with sanguinary vessels, and composed of many coats, the exterior of which covers the convex surface of the placenta.
A vitelline membrane is secreted by the inner surface, and a chitinous chorion by the outer surface of the egg-follicle.
The convex surface of the chorion is perforated by numerous micropyles, fine pores through which it is probable the spermatozoa gain access to the interior of the egg.
The chorion now forms a transparent yellowish capsule, which under the microscope appears to be divided up into very many polygonal areas, defined by rows of fine dots.
Dyar puts forward a classification founded entirely on the structure of the larva, while Tutt divides the Lepidoptera into three great stirps characterized by the shape of the chorion of the egg.
It is soon modified, however, as the villi on one part of the chorion are withdrawn; while on the other part they grow proportionately stronger, and unite intimately with the mucous membrane of the womb.
Each child has also a separate amnion, and waters; but there may be only one chorion and decidua for the whole, or each may have a complete set of membranes itself.
It is composed of one artery and two veins, twisted together like the strands of a cable, and of a sheath surrounding them composed of the chorion and amnion.
Cases every now and then occur where a considerable quantity of fluid is found between the chorion and amnion in labour at the full period of pregnancy.
The chorion is the proper covering of the ovum, and corresponds to the membrane lining the shell of an egg, in oviparous animals.
The placenta is formed essentially by the chorion and decidua; it is a flat, circular, or more or less oval mass, soft, but becoming firmer towards its edge.
Hunter, where he says, "these are the remains of those shaggy vessels which shoot out from the chorion in a young conception, and give the appearance of the ovum being altogether surrounded by the placenta at that time.
It is loosely connected with thechorion on its external surface, except when this membrane unites with the decidua to form the placenta at which spot it adheres to the chorion much more firmly.
The chorion undergoes various changes during the different periods of pregnancy, and forms a very important part of the physiology of utero-gestation.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "chorion" in a variety of sentences. We hope that you will now be able to make sentences using this word.
