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On the Causes which lead to the Attachment
of the Mammalian Embryo to the Walls of the Uterus.
Richard Assheton, M.A.
With Plate 19.
THE FIRST ATTACHMENT OF THE RABBIT EMBRYO TO THE
WALLS OF THE UTERUS. By the end of the eighth day, if not actually attached to the walls of the uterus, the embryos have become definitely located, and their presence is made evident from the exterior of the uterus by a bulging in the wall opposite the mesometrium. Frequently by this age they are actually attached to the uterus, and cannot easily be extracted without damage.
It must also be noted that the embryos by this time no longer lie anyhow in the uterus, but when definitely located the position of the embryo to the uterus is such that the embryonic area is always towards the mesometric wall of the uterus.
This is a very important fact. It is probably necessary for the development of the rabbit that it should be thus situated. It would be very awkward if the embryo became fixed in any other position. The shape of the uterus at this stage, and the shape of the blastodermic vesicle at this stage, are so beautifully adapted one to the other as to render any other position almost impossible. The blastodermic vesicle by this time is a slightly elongated body whose lower pole is semicircular in transverse section, while the upper pole is much flattened. If
a piece of uterus is blown out with water, then hardened, and a transverse section cut, the cavity will be seen to be bounded by a semicircular wall on the abmesometrial side, and, by reason of the two largely developed “placental" lobes, a very flattened wall on the mesometrial side.
Fig. 1, Pl. 19, is a figure of the section of the uterus during the early days of pregnancy, or in such part of the uterus in which no embryo is present during rather a later stage in the earlier part of pregnancy. M. is the mesometrium; C. is the cavity of the uterus.
The general external outline of the uterus is circular; so is the inner muscular system, but the latter is eccentrically placed to the outline of the uterus.
Within these muscular coats, which form the most resisting part of the uterus, comes the soft connective tissue and epithelial lining of the uterus. The loose connective-tissue layer seems to be the most yielding, and, as a consequence of the blastodermic vesicle within the cavity of the uterus, the folds or lobes, as they appear in transverse section (PL. L., PP. L., OP. L.), diminish in size, and as the body within increases in size, one by one they begin to disappear. The obplacental lobes have quite disappeared by the middle of the eighth day, and the periplacental lobes can no longer be described as lobes or folds after the beginning of the ninth day. The placental folds being the largest, and also upon the less expansible side of the uterus, become much flattened but never entirely disappear.
About the time that the blastodermic vesicle becomes de. finitely located, its shape is in transverse section as fig. 4.
The lining of the uterus is seen in fig. 1 to be thrown into folds. When the blastodermic vesicle (fig. 4) is inserted into the cavity, the folds become pressed out to a greater or less extent: thus the folds called obplacental are entirely obliterated (OP. L., fig. 2), the periplacental are nearly obliterated, but the placental lobes being very much larger, and also being supported by a larger mass of muscular tissue, are hardly compressed at all.
The cavity of the uterus is thus bounded on the mesometrial side by an almost straight line, while the opposite wall is circular. A body having the shape shown in fig. 4, in passing down the tube by virtue of the continual slow contractions of its walls, could hardly fail to become so fitted as to lie with its flatter surface against the flatter wall of the uterus.
Since the flatness of one surface of the blastodermic vesicle is caused by the embryonic disc and changes connected therewith being upon that surface, it follows that in the rabbit the embryo always comes to lie up against the mesometrial side of the uterus, which is, under the circumstances, by far the most favorable position for its future development. For in this position it will be less exposed to the tension which must necessarily arise upon the opposite side in the subsequent expansion of the uterus by the accumulation of fluid within the blastodermic vesicle.
The blastodermic vesicle does not by any means become attached with the centre of its flat surface always exactly adjoining the median cleft between the two placental lobes. It may sometimes be a little to one side or the other, but it is never very far out. As the blastodermic vesicle expands, it eventually fills up
the whole cavity of that section of the uterus in which it happens to be. With further expansion it necessarily exerts a pressure upon the walls of the uterus, and this pressure is made apparent without by a swelling, or protuberance, occurring upon the side of the uterus away from the mesometrium.
The first visible sign of the commencement of the formation of the placenta-or, at any rate, of those structures which eventually cause the placenta to come into existence, appears very shortly after the appearance of this swelling. I allude to the papillæ and protuberances of epiblast.
I said just now the first visible sign because I believe that the outgrowths are due really to a continuation of those causes which have given to the blastodermic vesicle its present form and shape, and described in a former paper.
The immediate cause of the origin of the papillæ seems to be
the pressure which now is applied to the vesicle from without by reason of the resistance offered by the elasticity of the walls of the uterus to the internal hydrostatic pressure.
First of all, let us consider what is the nature of these papilla-like growths.
Sometimes during the eighth day, it may certainly be as early as the seventh day four hours, when the uterus is swollen very considerably by the pressure of the contained blastodermic vesicle, here and there it may be noticed in transverse section that over all the lower surface of the vesicle certain of the epiblast cells are no longer so much flattened, but the nuclei appear rounded instead of oval in section, and the protoplasmic part of the cell much more distinct and granularaltogether more comfortable-looking (v. a., fig. 5). A little further along, at b., may be seen a couple of such nuclei in a mass of granular protoplasm. At c. a group of three or four, or more, of such nuclei in a mass of granular protoplasm. Outside this may be seen the torn remnants of the much attenuated albumen layer. The portion of the walls of the vesicle here figured was part of the lower pole of the blastodermic vesicle of an embryo of seven days four hours. Although I did not succeed in taking it out of its place in the uterus unhurt, it was, nevertheless, not yet attached to the walls of the uterus at any point. It may be noted that here there is no trace of an inner layer or hypoblast.
In fig. 6, another piece of the wall of a vesicle of about the same age cut in situ in the uterus, this piece shows a portion of the side of the blastodermic vesicle, or, at any rate, a portion not so far removed from the embryonic pole as that drawn in fig. 5. Here in fig. 6 the same features are to be seen in the epiblast as described for fig. 5. There is here, however, a well-developed hypoblastic layer.
Fig. 8 is a section of the upper part of the wall of the vesicle, of that part which closely adjoins the embryonic disc. This specimen is from an embryo from the same rabbit as that from which fig. 5 was drawn, and almost exactly the same size. Here it will be noticed that not only are the cells