The sixteen or seventeen days after fecundation, a rounded, whitish speak is perceptible on some portions of the blastodermic, vesicular membrane, and really stands out in relief; this is called the tache embryonnaire. This embryonic spot is composed of granulations, like those of the blastodermic vesicle, only they re more numerous and more concrete. It is very evident that the embryonic spot and the blastodermic membrane are each developed from processes of a double lamina, which may be separated by means of fine needles
Fig. 39 accurately portrays the doubling of the blastoderm; the embryonic spot assuming an elongated form. Finally the blastoderm exhibits a convex and a concave surface, a s in Fig. 41. The concave surface is divided into two distinct portions, one embryonic, the other becoming he umbilical vesicle. The tache embryonnaire becomes the embryo. The margins of the embryo exhibit a tendency to double over leaving a cavity (as well as the extremities) of considerable depth. The extremity which is most fallen is called the cephalic, the other the caudal extremity. As the development of the embryo goes forward, numerous minute elevations appear, scattered over the external surface of the ovum; these become the villosities which subsequently appear on the chorion. At the same time, the external layer of the blastoderm is raised in folds around the central or embryonic portion;see Fig. 42. Figs.43 to 46, exhibit the continuous approach of the embryo towards the centre of the ovum, until the blastoderm has become united over the dorsal surface of the embryo, their union absorbed, and the embryo, surrounded by a complete fold of the inner or mucous surface of the blastoderm, which seems to be a continuation of that upon the abdomen of the embryo.
This new membrane thus formed, is called the amnion, and becomes distended and separated from the external surface of the embryo by the amniotic fluid. The eternal or serous lamina now forms a ring or membrane by itself, which surrounds the whole. This contains the foetus develops into the child of nine months, is forced onwards to joint the vitelline membrane and chorion. Immediately after the to join the vitelline membrane and chorion. Immediately after the formation of the amnions, the doubling in of the cephalic and caudal extremities increases and forms more and more the abdominal cavity of the embryo, and finally a merc canal leading out of the abdominal cavity into a large vesicle, called the umbilical vesicle; and this canal, as it finally closes up, forms the umbilicus of he nine months child.
Fig 43. Fig.44.
Fig 43. O. The umbilical vesicle. I. the internal or internal; and E, the external layer of the blastoderm. E,. A portion f the external layer converted into the amnions. E. The embryo. C. The limit of he amniotic hoods. V. The vitelline membrane.
Fig. 44. This figure show the amnios almost almost completed, and likewise the origin of he layer o he blastoderm, or the non- vascular chorion. V. The vitelline membrane. C. The amniotic hoods ready to slow up. A. The allantois
In the cuts 43, 44, 45, 46, blood-vessels may be seen running from the embryo into this vesicle and again returning, one artery from, and two veins returning to the embryo, these are called the omphalomesenteric vessels. As this doubting of the embryo goes on, see Fig 44, we find a slight elevation springing up at he spot where the rectum and bladder are confound in the other earlier days of embryonic life, under he name of cloaca. This begins to take place about he time the embryo has already exhausted all the nourishment contained in the umbilical vesicle, and the embryo would inevitably perish at this stage from want of nourishment but for this wise provision of nature, b which a connections formed with the parent. For this little elevation rapidly extends to the villi of the chorion, passing out from the abdominal canal alongside of the umbilical vesicle, and is called the allantois. the allantois is composed of two arteries and is called the allantois. The allantois is composed of two arteries and one vein; the arteries arising from the primitive iliacs, carrying arterial blood of the embryo, seeking a fountain for allaying its thirst for material where with to sustain the little being over which is presides, for the blood always contains all the material elements for upbuilding and for repairing tissue.
These arteries seem to compel the growth of he allantois, and, with the accompanying veins, (for veins always accompany arteries,) plunge into and taken root in the villous coat of the chorion when the desired nourishment is found in the blood in he blood of the mother. This is quickly carried back by the vein to the famishing embryo and delivered in its bosom through the liver, as will be seen father on. In the subsequent pages also, its will be shown that the first circuit in the embryonic circulation constitutes the beginning of to the placental mass to be afterwards described.
Fig.45. Fig.46.
Fig. 45. This figure shows the rapid progress of the allantois, and how it spreads over the foetus, the umbilical vesicle, and the amnios. This later begins to unsheathe the pedicles of the umbilical vesicle, and that of the allantois in such a way as to form a commencement of the cord. According to some writes the vitelline membrane disappears more and more. O. The umbilical vesicle. E. The amnios. E. The external layer of the blastoderm. C,. The point where the two hoods come in contact. V. The vitelline membrane almost entirely atrophied. A. The allantois.
Fig.46. In this figure, the allantois has spread over the whole internal surface of he ovum, and but very slight traces are left of the continually between amnios and that part of the external layer of a blastoderm which formed the non-vascular chorion; the latter has a tendency to be confounded with the chorion, and the amnios encloses the umbilical cord more and more. O. The umbilical vesicle. E. The amnios. C. The point where the two hood are fused into each other, and form but a single membrane. E. The external layer of the blastoderm. A. The allantois. V. The vitelline membrane.
In some animals, and perhaps in the human females, the allantois in its development spreads out like an umbrella; but, for all practical purposes it is sufficient to describe it as taking root in a single villus of the chorion, and spreading out more and more as more ample means of supply are demanded till the full time is accomplished.
The closing up of he abdominal or umbilical concludes the amnios in close juxtaposition to the remaining minute stem of he umbilical vessels, -as seen in Fog. 46; and as this canal closes up, the amnions sheathes over the allantois and the small stem of the remaining umbilical vesicle, which vesicle is thereby forced out and is found finally to be a little yellow boy lying between the chorion and the common, next to the placental mass; and the allantois stem is found to be one and the same thing as he umbilical cord entirely sheathed over by the amnios.
With this description of the development of the allantois, concludes the principal parts of the ovum; which are therefore: 1, The embryo; 2, the liquid in which it swims; 3, the amnios, filled with the liquor amnii, and forming a sheath over the umbilical vessels; 4, the umbilical vesicle with the omphalo- mesenteric vessels still to be seen communicating with the embryo; 5, the allantoid vesicle; 5, the allantoid vesicle; 6, the space between the amnios and the umbilical vesicle, filled with liquid; and 7, the chorion, the outer envelope over all.
THE DEVELOPMENT OF THE DECIDUA-The decidua, from first to last, it simply the mucous membrane of he uterus, hypertrophied by the influence of conception and gestation. This hypertrophied development, which constitutes the deciduous characteristic, and previously stated, which constitutes the deciduous characteristic, as previously stated, has already commenced when the impregnated ovule is about to make its escape from heavier. So that upon its arrival in the cavity of the uterus, the ovule finds a soft, velvety bad in readiness top receive it. And as the mucous membrane of the uterus is continuous with the of the Fallopian tube, so in cases of extra-uterine foetation he deciduous development has sometimes, although no always, been found in the uterus; and in one case of development of the ovum in the Fallopian tube, referred to my Muller, ( “Elements, vol.ii., p. 1572, London, 1842) the decidua was observed both in the uterus and in the tube.
Into the vegetative bed above described, the ovule become fixed by the villi of he vitelline membrane (which becomes the chorion) taking root and growing into it in every direction. The decidua at the same time grows up all ovum,-which latter thus becomes implanted in a living cyst, attached to one portion of the wall of the uterus, usually that of the fundus. When thus encysted the ovum is not only completely covered by the mucous membrane but by means of the villi of it sown membrane the chorion, it becomes attacked to the decidua, growing into it whenever they thus come in contact.
