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expense of the yelk, which body furnishes the materials for the formation of the earliest and most fundamental parts of the germinal structures. It is remarkable that when Baer, repeating more carefully and skilfully the observations of Prevost and Dumas, first discovered the minute ovum of mammalia, he thought that this object corresponded to the germinal or Purkinjean vesicle of other animals, an error into which he could not have fallen, if the true office of the yelk, namely that of nourishing the germ in the first stages of its existence, had, at that time, been comprehended ; so important is it, if it be wished to obtain a clear view of the whole pro. cess, scrupulously to investigate the very earliest changes that occur.
The constituent parts just noticed are then met with in the ovum of all animals, whilst it is still contained in the ovarium; and, as far as the germinal vesicle and its spot are concerned, there is tolerable uniformity of character throughout, the principal difference in the several classes depend ing on the relative size of the vitellus. Those who wish to satisfy themselves of this identity, may compare the small, greyish, and incipient eggs seen in the ovarium of birds with those of mammalia. Up to this point all is certain and undisputed; but in most animals, and especially in the ovipara, the ovum, after it has quitted the ovarium, receives, in its transit through the oviduct, and before it enters the homologous part to the uterus, certain additions, which are most apparent in birds. In this class it is well known that besides the chalazæ, which are evidently a mechanism peculiar to incubating animals, the upper part of the oviduct secretes the albumen or white, and the lower part called isthmus placed just above the uterus, the membrana testæ or chorion. We shall not pause to show that in many other oviparous classes both of these superadditions are made to the ovarian ovum, but proceed to consider what takes place in the case of mammals. Professor Owen, in his account of certain uterine ova which he examined in some pregnant ornithorhynchi, varying in diameter from 2 to 3} lines, states that there were two distinct membranes; the outer one is called the “chorion," and resembles the cortical membrane of the ovum of the salamander, except that it is of a more delicate texture; the inner membrane is regarded by Mr. Owen to be the membrana vitelli. Lying immediately beneath the latter, there was a thicker granular membrane, which is analogous to the blasto-derma or germinative membrane ; careful examination showed that there was no decidua. It is also important to mention that, besides the yelk, which was of a yellow colour and quite distinct, a fluid matter was found between the cortical and vitelline membranes, thus occupying precisely the same space as the albumen in the egg of the fowl. This distinguished zoologist further remarks that the cortical membrane was certainly not the decidua, but that “it is certainly analogous to the outer tunic of the uterine ovum of the rabbit and bitch, which in them is gradually separated from the vitelline membrane by the imbi. bition of an albuminous fluid.”
It is our own conviction that what is here proved of one of the lowest of the mammalia applies to the whole class. It is no slight indication that the Fallopian tube is a real secreting organ, and not a mere passage of transit, that its mucous membrane in all the mammalia in which we have examined it, including the human female, is highly elaborated, forming in the outer two-thirds of its extent, a multitude of projecting valves ; it is 1817)
Ovum of the Rabbit and Dug.
further found that this canal presents, in its uterine portion, a peculiarity of structure as marked as that seen in the isthmus of the fowl, when contrasted with the upper part of the oviduct. Developments and modifications like these must be associated with action and modification of function; and justify the inference that, in all mammals, the ovarian ovum receives, in its passage through the tube, in the first place an albuminous matter or white, and subsequently a distinct envelope, the chorion. In considering this question, it must not be overlooked that, amidst various modifications and much confusion, the best observers, from De Graaf to Baer and Martin Barry, have distinctly found the egg, previous to its attachment to the uterus, enclosed in two membranes, of which the outer is affirmed to be the chorion and the inner the yelk-membrane, or as it is called in mammals, the zona pellucida. The conclusion then at which we have arrived is, that in all essential particulars, the ovum of mammalia is identical with that of oviparous animals; ar inference which is not only interesting in itself, but more especially because it gives an assurance that, in the subsequent development, a similar kind of identity will be preserved.
Having stated what we believe to be the view that is most in harmony with the general constitution and economy of the ovum, it is necessary to notice Dr. Bischoff's researches relative to this point. In his former work on the process of development in the rabbit, to which a prize was awarded by the Berlin Academy of Sciences, this excellent observer states that the ovum, in its passage through the Fallopian tube, receives a distinct albumen, but he contends that there is no cortical membrane or chorion surrounding it. In the work now before us, the Professor says, " the egg of the bitch, according to my observations, acquires no white either in the oviduct or in the uterus ; this well-ascertained fact distinguishes the ovum of this animal very remarkably from that of the rabbit, which, as I have shown, receives a very distinct layer of white in the ovi. duct, and possesses it also, in the beginning, in the uterus. In the rabbit this matter soon disappears, since it becomes blended with the zona pellucida and with this represents the outer membrane of the egg, which, in the dog, is formed by the zona alone. It is therefore clear that no particular importance should be attached to this production of the albumen; and, therefore, that this last can have no essential influence upon the formation of the ovum and its coverings."-(Entwickelungsgeschichte des Hunde-eies, p. 69.) As regards the formation of the chorion, Dr. Bischoff says, in another place, it results from the junction or blending together of the yelkmembrane with the peripheral part of the animal layer of the germinal membrane.-(L. C., p. 122). We can only repeat our dissent from the correctness of this account, and especially that which attributes the chorion to the zona pellucida, which seems to be, like its homologous part in the bird's egg, the vitelline membrane, namely, merely a retentive bag, and not an active structure.
In proceeding to notice more particularly the work of Dr. Bischoff, we would in the first instance observe, that it is a treatise to which, at this particular time, great value must be attached ; for, although much of the matter it contains is not novel, there are many facts recorded relating both to the earliest parts of the process, and to the subsequent changes taking
place in the uterus, which bear upon some of the most obscure but essential phenomena connected with the history of development, and which, at this time, engage a large share of attention of physiologists. In the second chapter, the author enters upon one of these difficult questions, the mode, namely, in which the semen operates on the egg. He justly remarks, what a reference to the writings of the older physiologists will amply confirm, that until lately the greatest doubt still prevailed upon the most important questions relating to the impregnation of mammals. In the following passage this uncertainty is well stated :—“ It was not known whether the formation and escape of the ova was or was not dependent upon coitus ; whether the semen of the male played a material, and what part; whether this fluid penetrated to the ovary, and in that place fertilized the egg, or whether both first met in the uterus or in the oviduct ; whether conception took place at the instant of connexion or subsequently; and what changes the ovum thereon experienced; all these questions were variously answered, and for the sufficient reason that there were no certain facts upon which to rest the reply.”—(L. C., p. 13.) At the present moment most of these doubts have received their solution; and it is not useless to add that this might long since bave been accomplished, if physiologists would have been content to receive information from the marked and ample phenomena displayed in the animal kingdom.
We need say nothing on the first question raised; because it is now ascertained that the maturation and discharge of the ova, are processes altogether independent of any influence communicated by the male; on this subject the researches of Raciborski, Negrier, Girdwood, Dr. Lee and others leave scarcely any room for doubt. With respect to the influence of the semen, the majority of our readers will recal the vague and frequently absurd theories, which some few years ago were put forth in works and lectures. And yet many accurate experimentalists, among whom Spallanzani, J. Hunter, Haighton, &c. may be mentioned, had distinctly proved that the act of impregnation depended on the material contact of the semen with the ova ; a fact equally substantiated by the mode in which the egg is fertilized in fishes and batrachia by the direct application of the seminal fluid. But all these and many other exact observations were insufficient to subvert the old doctrines of the aura seminalis, general sympathy, and other equally vague notions; “it was in fact,” to borrow the language of our author, " impossible to clear up all these involved questions, so long as the egg itself contained in the ovarium was not known; and since it has been discovered, no one has devoted a sufficiently careful and successful investigation to the subject.”-(L. c. p. 13). It is well known that Dr. M. Barry advanced the theory, that when an ovum was fitted for impregnation, it was carried up to the part of the Graafian follicle next to the outer surface of the ovary; that the vitelline membrane or the zona pellucida then in turn became attenuated, and that at length a fissure was formed in it, through which a spermatozoon penetrated to the germinal vesicle. Dr. Barry subsequently obtained the impregnated ova of a rabbit from the Fallopian tube, in which he supposed, as well as several other physiologists to whom the specimens were shown, that spermatozoa could be detected actually within the interior of the zona pellucida.
This view of the subject, which is supposed to be in harmony with the
7 known method of impregnation in the plant, where it is said the granules of pollen are conveyed down the style into the ovulum, has been to some extent received in this country; but according to Dr. Bischoff it is founded on a deception. He says, "I have stated in my work on the Development of the Rabbit, and have so depicted it, that the egg of that animal whilst in the Fallopian tube is covered with spermatozoa. I have even often seen them moving actively.” “ I am now certain that Dr. Barry has mistaken spermatozoa situated upon the ovum, for the same bodies within it.” Those who have themselves made observations upon ova taken from the tube, are aware of the great difficulty of deciding whether the spermatic animalculæ, which are, unquestionably, in immediate contact with the egg, are without or within the zona pellucida, and will therefore be inclined, for the present, to suspend their judgment upon this deeply interesting point of embryology. It is, however, most important to know that, whatever may be the solution of this question, it is well ascertained that the seminal fluid, as indicated by the presence of its animalcules, penetrates as far as to the ovarium, upon which it has been seen by Martin Barry and Bischoff in the rabbit. In the work of the latter, now before us, the following conclusions are given, after numerous observations, with respect to the dog :
“1. The semen penetrates at the instant of copulation through the os uteri into its cavity, and even as far as the furthest point of its horn.
“ 2. The semen in the subsequent hours penetrates also into the Fallopian tube, and can thus reach the surface of the ovary.
“ 3. The semen comes, in every instance, into material contact with the ova, and impregnation is effected by means of a material reciprocal action between the semen and the egg.
Impregnation never occurs at the moment of copulation, but at a later time, to be hereafter specified.”--L. C. p. 16.
Although there are no sufficient data at present, to enable the physiologist to ascertain the precise influence which the semen exerts on the ovum, it will still be useful briefly to consider this question, which however viewed is full of mystery. That the effect produced is most marked, is more particularly shown by the history of hybrids, and in a less striking degree by those physical peculiarities or resemblances which are impressed on the new being by the male parent : but on these points we do not propose to touch. There is no doubt that the semen operates upon the contents of the germinal vesicle, and whatever may be the subsequent (morphological) changes therein induced, the earliest ones seem to be of a chemical character; an inference which receives some support from the fact that, whereas the ovule of the plant prior to impregnation contains only starchy matter, it presents subsequently to that phenomenon another organic principle forming the walls of the newly-produced cells. This view corresponds with that advanced by Professor Bischoff, to the effect that the operation of the semen upon the egg is in the first instance of a chemical nature : and he further believes, with Valisneri, Bory St. Vincent, Valentin and others, that the object of the spermatozoa is to maintain by their motion the easily-changed composition of the semen.
As to the time and place of impregnation they are both subject to variation. The author concludes that, in the bitch, the ovum is capable of being impregnated, during the whole of the interval occupied in its transmission from the ovarium as far as to the end of the Fallopian tube, that is to say, from six to eight days; but when the egg has reached the uterus, it is no longer susceptible of the influence of the semen, inasmuch as the process of development commences, after conception, in the lower part of the tube. In support of this position, it is an interesting fact that in all animals subject to periodical venereal excitement or heat, this entirely ceases when the ova have arrived at the uterus. With respect to the place of impregnation, the principal point to determine is whether it is possible for ova to be impregnated whilst in the ovarium. If the records of extrauterine conception in the human subject are to be relied on, this may take place; for cases are referred to, among others, by J. Hunter, of ovarian pregnancy.*
It must, however, be confessed that the excellent researches of Prevost and Dumas, joined to the now ascertained fact that the actual contact of the semen is required, have created a rather general belief that the ovum escapes from the ovarium in consequence of the act of copulation, and that the semen coming in contact with it, either in the tube or in the uterus, impregnation takes place in one or other of these organs, and nowhere else. This opinion has been more positively affirmed in a late work published by M. Pouchet (Theorie Positive de la Fécondation des Mammifères, Paris, 1842). This writer says that physical obstacles are opposed in mammalia to the contact of the semen with ova still contained in the Graafian follicles; that assuredly ovarian pregnancies, properly so called, do not exist; and that, as fecundation in mammals, takes place nor
ormally in the uterus, abdominal and tubular pregnancies do not indicate that impregnation is effected normally in the ovary.
The observations of Dr. Bischoff on this question are so important and exact, that no apology is required for laying them before our readers in the author's own words. After stating that he has repeatedly found, in the rabbit and dog, spermatozoa in different parts of the Fallopian tube, between the fimbriæ, and upon the ovarium, this distinguished physiologist remarks:
“ All theoretical objections are entirely disproved by these direct observations. I have also further shown that there is not the least impediment to the penetration of the spermatozoa into and entirely through the Fallopian tube, the movements of those bodies and the contraction of the tube itself being quite competent to effect such a result. Again, as to the assumed difficulty of the impregnation of an ovum whilst still in the ovarium, they do not exist. I have expressed my conviction that the fluid portion of the semen is the fructifying part, and that the smallest amount of it is sufficient to produce the effect. There is nothing therefore in the way to prevent the semen penetrating through the coverings of the ovary and of the Graafian follicle, till it reaches the ovum ; especially if it be recollected that all these envelopes, at the moment when the
Hunter admits three forms of extra-uterine conception, according to the situation of the fætus, namely, ovarian, tubular, and abdominal; it is evident, however, that the latter is owing to the escape of the ovum by rupture or absorption from one or other of the former situations. Hunter justly remarks on the obscurity attending these cases. They are, he says, “ extraordinary and seldoin happen, and when they do occur, are often attended with so many hindrances to critical investigation, as hardly to allow of thorough or satisfactory information.”