Now as the ovum becomes developed, it is very evident that one portion of it must remain in contact with that region of the uterine wall to which it has already become attacked, and this, as as already become attached, and this, as already stated, is usually the fundus, the other portions of the ovum must therefore continually extend more and more into the cavity of the uterus, during the entire period of uteri-0gestation. In tracing the cavity of the uterus at an early period of gestation, we find something like four-fifths f the apical portion entirely unoccupied by the ovum. That portion of the decidua which covers this part of the interior surface of the uterus is called the decidua vera. This part of the membrane at the same time covers all the lover segment if the ovum, or chorion, and is therefore called also the epichorial decidua ovum and the portion of the wall, or fundus of the uterus to which the ovum itself is more immediately attacked, is called the decidua vera. This part of the membrane at the same time covers all the lower segment of he ovum, or chorion, and is therefore called also he epichorial decidua. That part of the deciduous membrane which extends between the ovum and the portion of the wall or fundus of the uterus to which to ovum and the portion of the wall, or fundus of the uterus and upon the chorion of that side. As the development of the ovum advance, s the epichorial decidua is pushed farther and farther into the cavity of the uterus, towards its apex, and of course grows thinner and thinner. The villi which at the outset so thickly studded this portion of the chorion become gradually atrophied for want of use; while at the same time those of the opposite side become more and more largely developed, from being the medium through which supplies are transmitted from the mother to the embryo. Finally, a t the close of the period of utero-gestation, the epichorial decidua is found to have been forced down to the very apex of the uterus, and thus everywhere brought into contact with the decidua vera. The utero-epichorial decidua remains as it was. The whole however becoming so worn out, having performed its uses, that it is cast off at parturition, and replaced by a new membrane.
THE FURTHER DEVELOPMENT OF THE OVUM.-Having thus described the provision made for the primary reception and support of the ovum in the changes which occur in the interior surface of the uterus, we will now proceed to the more particular examination of the further development of the ovum itself. We have briefly traced this development from the original egg to the production of the embryo; and noticed in a cursory manner the umbilical and allantoid vesicles, the amnios and the chorion. These appendages to the embryo, which at the same time protect it and administer to its growth require now to be more particularly considered.
The allantois, as before partially described, is usually observed to arise as a minute tubercle from the inferior portion of the canal, about the tenth day it then rapidly shoots forward and takes root in the villi of the chorion. This organ is also called the urachus; and the villi of the chorion. This organ is also called the urachus; and is accompanied by, or rather principally composed of two arteries proceeding from the iliacs and called the umbilical arteries, and one vein which returns the blood from he mother to the embryo
The allantoid vesicle, as such, disappears very rapidly; after a few days no trace of it can be found, excepting a cord of no definite length which connects the embryo with the chorion, and contains the umbilical vessels. That portion of this vesicle which is contained with the abdomen of the child, is, however, more persistent in it duration, becomes converted into the urinary ladder, and in the rudimentary state terminates in the rectum and constitutes the temporary cloaca, all of this is capable of demonstration in the human subject. It is here easy to understand what is meant by the urachus, which is really that portion of the allantois which extends from the rectum to the umbilicus; hence and thing that is capable of causing descent of the uterus must drug on the bladder from this attachment, and the result the dragging sensation felt in the umbilicus is such cases, from the attachment of the urachus to that organ.
The umbilical vesicles, when first seen, seems to occupy the whole of the cavity of the ovum Subsequently the embryo is seen on the blastoderm, into back corresponding to the serous, external, surface. and its abdomen to the mucous or interior surface of the blastodermic membrane. Thus at this early period the abdomen is open to the entire umbilical vesicle. As development goes on he embryo seems to rise more and more towards the umbilical vesicle; and to the developed forwards and inwards upon he abdomen. In consequence this vesicle loses more and more of its bulk and assumes he appearance of he long narrow stem. This vesicle contains a yellowish and highly nutritious fluid, which, through the intervention of the vascular apparatus, serves to nourish the embryo, until it can provide for itself by other means. This vascular apparatus is supplied with two trunks for the transmission of blood, one venous, the other arterial both however accompany the pedicle and form an important constituent part of it. See Fog. 60, Plate III. “The first, N, called the omphalo-mesenteric vein, enters the abdomen, winds around he duodenum, a nd then opens into the umbilical veins at the point I, just as the latter emerges from the liver. As it passes the duodenum, branches are gives off to the stomach and intestines and when it liver. That portion which furnishes the branches just described, whilst all he rest will disappear with the umbilical vesicles nd its pedicle. The arterial trunk P accompanying the pedicle, has been designated as the omphalo- mesenteric artery. Arising from the aorta, it gains the summit of the intestinal convolution, and gives off branches to the mesentery and to the intestine itself; then it reaches the pedicle,, and follows the latter to the umbilical vesicle upon which it ultimately ramifies. The part that supplies the mesentery is converted in the adult into a mesenteric artery, all rest being effaced. From all which, it appears that the vascular system of the umbilical vesicle represents the primitive circulation in the embryo, corresponding in it to the sanguiferous apparatus of the yolk of fowls. Cazeaux.
The amnion, as before stated, becomes developed by the embryo rising into the umbilical vesicle, thereby completely developing the blastoderm about itself, when the doubled parts unite, the bridge uniting this new ring with the former becomes absorbed, leaving tow distinct circles formed out of one, the former being the chorion the latter the amnion. Now as the amnion become distended more and more, with liquor amnii, of course the umbilical vesicle becomes encroached upon and grows smaller and smaller, at the same time the abdomen of the embryo cures up more and more, and the pedicle of the umbilical vesicle becomes longer and smaller, being now found on the outside of the amnion, between it and the chorion. At the end of six weeks after conception this is seen to be a small yellowish point about as large as a coriander seed. The umbilical vesicle is of vital importance to the embryo until after the formation of the allantois and its union with the villi of the chorion, after which it is no longer of any particular account, and with the exception of the above-mentioned remnant of its artery and vein, it becomes entirely atrophied.
The amnion is thus seen to be the most internal membrane of he body, and that it is formed by folding the blastoderm over the embryo in every direction, which later is accomplished apparently by the rising of the embryo towards he centre of he umbilical vesicle. In this manner the external or serous surface of the blastoderm is turned inwards and forms the inner surface of the amnion, and the internal or mucous surface is turned outwards. When the folding over is complete, and both portions have grown together so as to form a complete shut sac around the embryo, then we have the amnion containing the embryo, the umbilical vesicle, and a quantity of fluid, thick and gelatinous between the amnion and the chorion, which becomes less and and less as the amnion that the amniotic fluid is a secretion from the surface of the amnion is a mistake, since this liquid is unquestionably an efflux from the embryo itself. From the moment of the complete formation of he amnion and shut sac, there is not longer any outlet for the escape of the efflux from the embryo itself. From the moment of the complete formation of the amnion as a shut sac, there is no longer any outlet for the escape of the efflux from the embryo; and that there must be such an efflux, must be evident since there are certain portions of the nourishment flowing into the embryo that are not entirely assimilated, some material carried the either from the mother, which although it has served the purpose for which it had been imbibed, must be in part rejected in the form of sensible or insensible perspiration. In fact there can be no doubt that there is enough of what we may call offal from the embryo and foetus to account for the quantity and quality of the liquid found within the amniotic membrane.
