« ΠροηγούμενηΣυνέχεια »
Office of the Germinal Vesiele.
orum is on the point of escaping, are thinned to the uttermost. It is, therefore, certain that eggs still in the ovarium can be impregnated; but this does not altogether prove the possibility of their becoming developed in the ovary, or of ovanan pregnancy, a phenomenon which seems to rest on incorrect and inaccurate observations.
He then adds that ovarian conception is, however, only the exception ; for as the ova quit the ovarium, not in consequence of the influence of the semen, but of their being fully developed, and as in general this happens before the seminal fluid has had time to reach the ovary, the rule is that " the ova and the semen meet together in the Fallopian tube, and that here impregnation takes place."-L. c. p. 30.
In this extract we have no doubt the phenomenon relating to the seat of impregnation are correctly set forth, excepting that it is with us a question, whether, in ovarian conception, the semen does not come into direct contact with the ovum, the Graafian follicle, whilst grasped by the fimbriæ of the tube, being as usual ruptured, but the egg, subsequently to the act of fertilization, being by some unknown cause retained within the ovi sac, and there becoming developed.
Although, as we have said, it is not our object to trace the process of development, it will be desirable to notice some of those parts of it, concerning which at present there prevails much uncertainty. One of these relates to the first changes that ensue in the ovum consequent upon its fecundation; and this should be a subject neither for surprize nor discouragement when the minute size of the objects to be examined are recalled to mind, the ovum of the bitch having, when it quits the Graafian follicle, a diameter not exceeding the t'o or is of a Paris line, whilst that of the germinal vesicle is pretty constantly in the dog só, and of its macula zoo of a line. Those of our readers who have kept up with the progress of modern embryology, will recollect that all investigators are agreed in attributing to the minute vesicula germinativa, and even to its spot or nucleus, some predominating influence in the first acts of development: it is even thought and apparently proved, that the basis of the embryo is produced by the direct operation of this fundamental element of the ovum. But, although there is this accord with reference to the supreme importance of the Purkinjean vesicle, there is the greatest discrepancy as to its mode of action. These conflicting views may, however, be referred to two hypotheses : according to one, and it is that which, resting on the deeply interesting researches of Dr. M. Barry, is rather generally adopted in this country, the vesicle receiving the direct influence of the semen (according to Barry a spermatozoon penetrates into it), takes on a complex series of cellformations, determined especially by the reproductive powers of the macula germinativa, and which ends at length in forming the first rudiments of the germ, so that, in this theory, the vesicle is not only the determining cause of the primary acts of development, but also the seat of them, the yelk thus performing the merely subordinate part of furnishing by endosmose the crude nutritious matter.
According to the second, more generally received, and, as we believe, the true hypothesis, the vesicula either is ruptured or dissolved as the immediate and necessary result of a fruitful copulation; it thus disappears as a vesicle, though the macula or nucleus germinativus may remain intact, and play a very active part in the subsequent evolutive process. This is the account of the matter to which Professor Bischoff inclines in this and his earlier works; he having always contended, sometimes we think with unjust asperity, against Dr. Barry's peculiar views. The series of actions consequent upon this rupture of the germinal vesicle has been variously regarded; but there is much concurrent testimony, and we allude herein more particularly to the earlier researches of Prevost and Dumas, to the subsequent ones of Rusconi, Baer, Rathke, and Siebold, and to the still later ones of Kölliker, Bagge, Vogt and others on the remarkable process called the cleaving of the yelk ; there is we say much concurrent evidence to show that the way in which the contents of the vesicle conduce to development is by determining the subdivision of the vitellus into minute segments or globules which are ultimately converted into nucleated cells, and that these united together constitute the gerininal membrane or true rudiment of the embryo. Thus it is found that, after impregnation, the yelk has in its centre a single cell (the first embryonic cell according to Kölliker): this cell next divides into two secondary cells, and at the same time, and apparently as a consequence, the yelk also breaks up into two halves ; and this process of subdivision goes on in arithmetical progression, till at last a mulberry-shaped mass is produced lying of course within the zona pellucida or vitelline membrane. In the bitch, according to the au. thor, the ovum at the end of the Fallopian tube has between 16 and 32 of these segments; and in the centre of each he found a very delicate vesicle, powerfully refracting the light.-(L. c. p. 44). There has been much difference of opinion as to the exact relation which the segments of the yelk and the central bright vesicle have to the cell-formation; but, without dwelling upon this involved point of minute anatomy, it will suffice to state the general conclusions of Professor Bischoff on this subject. He says " these yelk-globules are not cells, but an agglomeration of yelk. granules without an envelope ; each contains in its interior a transparent vesicle, similar to a fat vesicle, but without a nucleus, As to what determines this process of cleaving, and what is the source of the transparent vesicle, nothing is still certainly known ; but it appears that the cleaving of the yelk and of its segments, is dependent upon these vesicles, and that these take their origin either from the germinal vesicle or from its nucleus.” “ In the uterus, the ovum has at first still the same appearance as in the oviduct, and the subdivision of the yelk still proceeds. But now the yelk-globules, continually diminishing in size, transform themselves into cells by becoming surrounded with a delicate membrane. The nuclei of these cells are the transparent vesicles placed in the centre of the globules. These cells arising out of the yelk-globules rapidly join together, and, their number still increasing, represent, by becoming flattened and blended together, a very delicate membrane lying close beneath the zona pellucida, which thus appears as a vesicle, called by me the vesicula germinativa."-L. c. p. 119, 120.
Such, then, are the earliest changes in the fecundated ovum, by means of which the first rudiment of the embryo is laid down in the form of a delicate cell tissue, commonly called the germinal membrane, but as we have just seen named by the author, germinal vesicle. We shall not pursue the subsequent changes of development, but proceed to notice what
Weber on the Uterine Glands,
has more interest for the medical practitioner-the mode of attachment of the orum to the uterus, and the formation of the placenta.
There can be no hesitation in affirming that, of all the branches of embryology, by far the most difficult in its investigation and the most involved in its description, is that which concerns the connexion existing between the uterus and the fætus; and, notwithstanding the numerous and valuable contributions made of late years to this important subject, if we may judge from some of the publications that have appeared in this country, and from conversation with many of our professional brethren, very vague notions still prevail even among professed accoucheurs. The time, however, is arrived when the obscurity and contradictions in which the description of the placenta has been involved, may be all but completely removed; and it will be our object, with the aid of the recent researches of Professors E. H. Weber and Bischoff, combined with those of other observers, to give a clear and succinct account of this matter. To do this it will, as in all similar cases, be necessary to consider the earliest changes in the uterus consequent upon conception, and the first relations established between that organ and the ovum. It is well known that, as the result of impregnation, there is formed in the uterus the production called des cidua; and that, under this name, two substances are comprised, one lining the inner surface of the uterus being called the decidua vera s. uteri, and another investing the ovum, called the decidua reflexa. Now, until the researches to which we shall immediately refer, these two productions were generally supposed to be both formed by a coagulated lymph secreted by the mucous coat of the uterus; but it is a point of primary importance to know that this is not the fact. The two tunicæ deciduæ are perfectly distinct structures; one, the decidua vera, is nothing else than the developed and thickened mucous membrane of the uterus; whilst the other is unquestionably a true secretion, derived in all probability from the uterine glands. For the first positive step in the right direction, and by which a beginning was made for the reversal of the generally received Hunterian doctrine on the production of the decidua, we are certainly indebted to Professor E. H. Weber, who, in his edition of Hildebrant's Anatomy, minutely and accurately described in ruminants the glands of the mucous membrane of the uterus which he had already seen in the uterus of a woman, impregnated seven days previous to death, and which, on account of their form, he named “ glandula utriculares.” He also especially pointed out the manner in which the cells of the chorion take up the yellowish fluid poured out by these flask-shaped glands.
The existence of these most important glands seems to be general among mammalia : thus they have been discovered by Eschricht in the porpoise and cat; in the rabbit by Weber; in the bitch by Dr. Sharpey ; and in the human uterus by Professor Weber, and independently, by Dr. Sharpey, to whose admirable description in Dr. Baly's translation of Müller's Physiology we have much pleasure in referring our readers. On the present occasion we shall principally avail ourselves of the recent observations of Professors Weber and Bischoff, which appeared in Müller's Archives for 1846.
Professor Weber states that, after conception, the mucous membrane of the human uterus becomes soft, and, by degrees, attains a thickness of two or three lines : it is this structure which forms the tunica decidua uteri. This change depends upon the increase partly of the vascular and partly of the non-vascular layer of this membrane or its epithelium ; that is to say, the blood-vessels of the mucous membrane of the uterine glands become enlarged, and, at the same time, an abundant formation of white epithelial cells, partly nucleated, takes place. The utricular glands thus enlarged are two or three lines in length; they run like the gastric glands towards the inner or free surface of the mucous membrane, and there terminate by orifices, which have long been known as existing in the tunica decidua, to which they give a cribriform appearance.
In the dog and cat the uterine glands, which in these animals are of two kinds, simple and large or branched glands, enlarge themselves in like manner at the place where the placenta subsequently appears; the smaller ones in their whole length, but the larger branched ones only towards their orifices, where they become dilated into a number of sacks or pouches, around the walls of which the uterine blood-vessels are dispersed, and which, as they enlarge, push as it were against the lining membrane, carry this before them in a complex manner, and thus become enveloped by it in the same way as the large intestine is covered by the peritoneum.
The fact that the decidua vera is composed actually of the developed uterine glands, and not, as the Hunterian doctrine taught, of an exuded fibrine, must now be regarded as firmly established ; and this, which is the clue to the whole formation of the placenta, requires us in the next place to consider what is the nature of the connexion established between the outer membrane of the ovum, the chorion, and the so-called deciduous membrane. In the ruminantia there is apparently afforded the type of what, under different modifications, takes place in all mainmalia : in these animals it is found that the tufts and villi of the chorion are received into the glandular tubular canals of the maternal cotyledons, just as the fingers are introduced into a glove. The connexion consists of intimate contact, not of union, and so the tufts of the fætal cotyledons can be easily drawn out-in the gravid uterus of the cow, for example—from the sheath of the maternal ones, when it is seen that they were bathed in a milky kind of liquid. A similar relation, in principle, has been detected by Drs. Sharpey and Weber in the bitch ; the latter thus describes it. “ The tufts of the chorion, which carry the embryonic capillary vascular plexus of the umbilical vessels, grow, as it were, into the expanded openings of the uterine glands ; fill out the expanded part of these glandular flasks; and, insinuating themselves into all their folds and tufts, grow with them, and thus form together a particular membrane, which alone possesses embryonic vessels. In this process of development, the part of the membrane derived from the walls of the uterine glands becomes probably thinned by absorption.” This important and elaborate process is confined to the dilated pouches of the uterine glands, as the tufts of the chorion do not seem to penetrate into their tubular portion, nor into their cæcal extremities.
The exact structure of the zonular placenta of the bitch produced in the way just described, is thus explained :--the whole placenta is penetrated or traversed throughout by a coarse network of tortuous capillary ves. sels, carrying the maternal blood, and having the very large diameter of
Structure of the Placenta in the Dog.
from about to Paris line; these vessels are each of them overlaid or covered by a membrane, which gives transmission to a very rich network of extremely minute embryonic vessels, the diameter of which being 17 to at of a Paris line, is three times smaller (giving an area nine times smaller) than that of the maternal blood-vessels which are drawn over them. Thus it happens that the embryonic blood contained in rich vascular retia, flows over the surface of the large canals carrying the maternal blood, but evidently in such a manner that the two kinds of vessels and their contents do not communicate together; they therefore only come into indirect but most extensive relation with each other, the disposition being similar to that of the small branches of the air-tubes with the rich networks of the pulmonary capillaries displayed over them.-Müller's Archiv.
These investigations into the structure of the placenta in the dog are so important in themselves, and supply so many of the voids at present existing in the human formation, that we are tempted to extract the very clear account given by Professor Bischoff as the result of his researches.
* Until the ovum has acquired the size of from 2 to 24 lines, and when it has still do villi on its surface, it lies quite free in the uterus, and no other change is seen in the mucous membrane of the latter organ, except that it is more vascular, turgescent and villous than usual. But when the ovum has attained the above size, the mucous membrane, at the place where the placenta is subsequently formed, rapidly develops itself, and soon forms a distinct elevation on its inner surface. If the free surface of this be now carefully observed, there will be seen, even with the naked eye, a great number of small apertures, and soon when the ovum with its tuft and the above elevation have increased in size, we can perceive that the tufts of the chorion project into these apertures : (this disposition is beautifuliy illustrated in the accompanying plates, fig. 48, A.) In the beginning, the tufts of the chorion can, after a slight maceration, be readily drawn out of the apertures; in a short time, however, this cannot be so easily effected, but” (and to this point we would direct the particular attention of our readers) “we can now much more readily accomplish a separation of the whole swollen part of the mucous membrane of the uterus, which remains attached to the ovum; the separation of this layer takes place so much the easier as the egg is more developed. Thus is formed the placenta of the dog. If a section of the mucous membrane be made at the zonular swollen place above described, it will be found that the swelling consists partly of a succulent infiltration of the whole tissue, but more especially of the above described uterine glands greatly developed : the small apertures noticed above as being seen on the free surface are the orifices of these glands, into which sink the tufts of the chorion."
Although the subsequent changes are difficult to decipher with accuracy there is no room to doubt that they consist essentially of the continued progress of the same disposition ; that is to say, the uterine glands increase more and more, and with them the tufts of the chorion, which, sending off numerous lateral branches and offsets, project into them as into sheaths. The umbilical blood vessels of the fætus ramify in the tufts, the arteries passing over into the veins in loops ; and in a similar manner the bloodvessels of the mother expand themselves between the uterine glands, a network of capillaries forming the connexion between the uterine arteries and veins. The whole forms the zonular placenta, in which therefore can be distinguished a maternal part, formed especially of the greatly developed uterine glands and uterine vessels ; and of a fætal part composed of the tufts of the chorion and the umbilical vessels ramifying