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ganised particles are in such quantity as to render the yolk quite opaque; but in a few animals, as osseous fishes, the clear fuid preponderates, and the larger or smaller globules are seen floating freely in it, while the germinal mass is confined to a particular region of the cavity.

But the most important distinction to be observed in the nature of the yolk substance, in the three groups of animals referred to, is connected with the relation subsisting between this substance, or some part of it, and the formation from the previously unconnected elements of the yolk of a united layer of organised matter out of which the embryo takes its first origin; more particularly the process of yolk-cleavage or segmentation, or, as it ought perhaps rather to be designated, germ-cleavage, which immediately succeeds to the fecundation of the ovum, and is the antecedent of the first embryonic genetic process. In the group of mammalia, the whole mass of the finely granular yolk-substance undergoes the process of segmentation, and contributes directly by that process to the production of the organised materials of which the embryo and its first accompanying structures are formed; the whole, therefore, of the yolk substance is directly germinal, without distinction of its parts, excepting as respects the position of the embryo in relation to the axis of the ovum and seat of the first cleavage. In the group of birds, scaly reptiles, etc., the small light-coloured disc or cicatricula situated on the upper surface of the larger yolk, and composed of the finely granular substance, is alone the seat of cleavage, and alone takes immediate part in the genetic process preceding embryo formation ; the remainder of the yolk, comprising the larger mass of deeply coloured substance, together with the lighter coloured substance of the centre or yolk cavity, and which consists of larger corpuscles of both to sooth of an inch in diameter, and presenting some of the characters of organised cells, is not immediately concerned in the formative process, but contributes only secondarily and indirectly to the subsequent formation of the organised parts of the embryo and its dependencies. This latter part may be looked upon, in fact, as an accessory supply of nutriment for the embryo after it has taken origin from the germinal part, constituting thus the nutritive yolk.

In the group of amphibia and osseous fishes the structure and relations of the yolk substance are in some measure intermediate between those of the other two groups, and the cleavage or first organising process affects a variable extent of the yolk substance; in some, as osseous fishes, scarcely more than a sixth ; in others, as alytes obstetricans, about a half; and in others, as the common frog, toad and newt, nearly the whole, but yet in such a manner that the segmentation is not in any complete, but affects more fully that portion of the ovum on which the germ is situated. A variable extent, therefore, of the yolk is germinal or formative.

The distinction now indicated between a germinal or formative, and a nutritive part of the yolk has long been known to physiolo100 PROFESSOR ALLEN THOMSON ON THE COMPARISON [FEB. gists in a general way; but recent inquiries have added greatly to the precision of their knowledge of its relation to the structure of the yolk. It has been well described in several of its aspects by Barry, Reichert, and Coste. We shall presently see that the observation of the earlier stages of formation of the ovarian ovum in birds and mammals tends to place in a still more prominent point of view the distinction between the germinal or formative and the nutritive substance of the vitellus.

3d, The external vesicular covering of the ovarian orum of animals has in general been named the vitelline membrane, from an assumed analogy with the covering of the yolk of the fowl's egg. In mammalia, however, an exception has been made to this, and the peculiar thickness and dense consistence of this membrane, and the doubts that have been entertained as to the true relation of the mammiferous ovum to that of birds and other animals, have led to the very general adoption of Von Baer's designation of zona pellucida, as applied to the external covering of the minute mammiferous ovum, when it leaves the ovary

In mammalia, as already remarked, the zona closely surrounds a yolk which is entirely germinal, and finely granular; and the membrane appears to take its origin at a very early period of formation, by the consolidation of the external portion of the clear substance in which the yolk granules are deposited. But in birds and other animals of the same group, the vitelline membrane of the mature ovum is a delicate homogeneous structure, enclosing the nutritive as well as the formative parts of the yolk. In the fully ripe state, it is almost entirely without visible demarcation of structure in its substance; but in less advanced ova, as, for example, in ovarian ova of the fowl of a quarter of an inch in diameter, we find it to consist in part of distinct cells united by their edges, and it is obvious, therefore, that the vitelline membrane of the fowl's egg is very differently formed, and has altogether different relations from that of the mammiferous ovum."

With this preliminary statement of the principal varieties in the form and structure of the essential parts of the ovarian ovum, I return to the consideration of the question, What is the true relation subsisting between these parts in the ova of the oviparous and vivi. parous groups, upon which physiologists have hitherto been in some doubt ?' I ought, however, to state here, that although the views of physiologists have differed on this subject, some have approached very near the correct view of its bearing; and among these, I ani happy to quote the opinions of Dr Martin Barry, as expressed in a postscript to the second series of his Researches on Embryology (Philos. Trans.' 1839, pp. 369, 370), and thus to repair what has appeared to Dr Barry to be an omission on my part, in the article Ovum recently published. In the memoir quoted, Dr Barry, after referring to the facts, that while segmentation affects the whole yolk of the mammiferous ovum, it occurs only in a limited part of that of the osseous fishes, puts the question, “Is not the discus vitellinus in the ovum of the bird, the seat of similar divisions ?” A remarkable anticipation, certainly, of what has since been proved to be the fact, by Bergmann, Coste, and myself; and he very justly continues: “If so, it will perhaps appear that the so-called yolk-ball in the mammiferous ovum, corresponds more particularly to the discus vitellinus (with its germinal vesicle) in the ovum of the bird.”

1 It is not my intention at this place to enter more fully into the consideration of the structure of the vitelline membrane in general. Recent interesting observations have pointed out the existence in that membrane in the ora of some animals, of peculiar apertures, either of a permanent or temporary kind, and apparently designed to facilitate the process of fecundation. My present object is limited to the attempt to show the true relation of these membranes (zona and vitelline covering) to the germinal and nutritive yolk.

With this view I entirely coincide, and the observations to which I am about to call the attention of the reader, will afford the most satisfactory proof of its correctness, though perhaps in a somewhat different manner than was anticipated.

In February 1852, Dr H. Meckel, of Hemsbach in Halle, published in the Zeitschrift für Wissenschaftliche Zoologie (vol. ii. p. 420), an account of some interesting observations on the formation of the ovum in birds, from which it appeared, that in the early condition of the ovarian ovum of birds, while its structure resembled more closely that of the mammiferous animal, its finely granular or primitive yolk substance is immediately invested by a covering corresponding to the zona pellucida of the mammiferous ovum; but this covering is only of temporary existence, subsequently disappearing on the formation of the cellular or nutritive yolk; and the whole yolk, that is, the larger mass of highly coloured or cellular vitellus, together with the primitive granular yolk substance, now disposed in the form of the vitelline disc, and containing the germinal vesicle, is at a later period enclosed by a membrane of newer formation, that, viz., to which the name of vitelline membrane is usually given in the bird's egg. H. Meckel farther showed, that the cellular part of the yolk of the bird's egg proceeds from the cellular contents of

While I gladly avail myself of this opportunity to quote the above view's of Dr Barry, of the omission to do which in my article Ovum he complains in a short paper published in the Philos. Magazine for October 1854, I must observe that Dr Barry has misunderstood some part of the remarks made by me at p. 77 of that article, and that more particularly he is in error in supposing that the remarks of mine which he has extracted and placed in parallel column with the statement of his own views, as above quoted, are tlie statement of H. Meckel's new observations on the formation of the ovum. The remarks so quoted were intended to express my own views of the relation of the parts of the ovum to each other, previous to the statement of H. Meckel's newer observations, which follows along with my own. Having with others long entertained these views, I did not feel myself called upon to give a historical relation of opinions in connection with this subject

the ovarian capsule, and that a part of these cells continues to the last to surround the vitelline disc, as well as the other parts of the yolk substance; but very unfortunately for the recognition of the value of these observations, and of the deductions to be made from them as to the morphology of the ovum, H. Meckel was led to adopt some untenable views of the relations of the several parts in the bird and maminal. More especially, he appears to have fallen into error in regarding the yellow or cellular yolk-substance of the bird's egg, as homologous with the corpus luteum of the mammal, instead of, as Von Baer rightly thought, and as seems more simple and natural, with the tunica granulosa or cellular contents of the Graafian follicle previous to its rupture.

Having confirmed and extended H. Meckel's observations on this subject, in the fowl and several other birds, I have thought it proper to call the attention of ovologists more particularly to them in this paper, especially as their correctness has been called in question by one of the most accurate and learned recent writers on the history of the ovum. In the article Zeugung in R. Wagner's Handwörterbuch, Professor Leuckart states that he has not been able to perceive the distinct zona pellucida described by H. Meckel, and he therefore refuses his assent to the view of the morphology of the ovum, founded on the existence of a zona enclosing the primitive yolk of birds; but he adds,” that were the existence of such a membrane proved, a very different view might be entertained from that adopted by him, and generally received by physiologists. It does not appear superfluous, therefore, to bring forward a statement in confirmation and extension of H. Meckel's observations.

In birds, as in mammals, and indeed in most other animals, soon after the germinal vesicle is distinctly seen within the simple ovarian follicle (ovisac of Barry) two additions to the structure take place, the one more immediately surrounding the germinal vesicle, the other more closely connected with the inner surface of the ovisac. The first of these is connected with the deposit of the primitive volk substance; the second with the formation of the granular cells of the ovarian follicle. In most animals the primitive yolk appears to consist of a somewhat clear basement substance, probably albuminous in its nature (see plate, figure 1st, y); and in this, either from the very first or very soon after its appearance, small dark or highly refracting granules, probably of an oily nature, are deposited. These granules are in greatest quantity near the surface of the germinal vesicle, and generally leave more or less of the clear fluid or basement substance without granules externally. This external clear part is not at first a membrane, as it has been described by some; but after a time it undergoes consolidation, so as to form a vesicular membrane or covering of the parts within. In mammalia

1 Vol. iv., p. 792. Victor Carus agrees with Leuckart on this point, (see his System der Thierische Morphologie, p. 186).

2 Ibid., p. 818.

this covering, which is consolidated at a comparatively early period, is at first thin, it gradually acquires greater thickness and tenacity, becomes soon capable of being detached from the granular or opaque yolk substance, and then constitutes the zona pellucida, or permanent external envelope of the ovarian ovum of these animals, In many invertebrate animals, the external covering of the yolk takes its origin in the same manner, and is to be regarded as the same with the zona, though great differences afterwards arise in its extent, thickness, and structure. In Batrachia and Osseous fishes I am inclined to believe the membrane of the yolk takes its origin in the same manner; in the latter animals this membrane, which has generally been called chorion, undergoes considerable changes in the progress of the formation of the ovum, being, in many, perforated with a number of minute canals through its whole substance.

In birds and scaly reptiles a similar clear marginal space occurs round the priinitive yolk (see figure 2d, z), and this has sometimes been described and figured as the vitelline membrane (as hy M. Barry, 1st series, figure 26), but this space never attains to the same degree of clearness that is perceived in mammalia, and it never forms a membrane of the same firmness with that which is called zona pellucida in these animals. Nevertheless, according to H. Meckel's discovery, which I have fully confirmed, when the ovarian ovum of the bird, consisting as yet only of the germinal vesicle and primitive or granular yolk substance, has attained the size of Loth or both of an inch in diameter, there exists all round the surface of the granular or opaque yolk, a much clearer ring of substance, of considerable thickness and some degree of solidity, which maintains its shape, and may be isolated under pressure, and which has the same relations to the other parts as the zona pellucida of the mammiferous ovum. My observations differ from those of H. Meckel only in respect to the amount of clearness of the substance of this zona, the smoothness of its outline, and the degree of its separability from the granular yolk substance. It appears to me to have more the appearance of the superficial part of the basement substance of the yolk, left clear, but not altogether transparent, but rather turbid or minutely molecular in its aspect, and adhering to the opaque granular yolk substance, and moving with it, and therefore not so separable or so distinct from that substance, nor so like a membrane with a well defined outline, as H. Meckel's description would lead us to believe (see figure 3 and 4, z.) It has, however, considerable consistence, and its relations are so entirely analogous, that I cannot but regard it as essentially corresponding with the zona pellucida or primitive vitelline membrane of mammalia.

This last is, in fact, its proper designation. It appears then that the first forined yolk in mammalia and birds is finely granular, and that the primitive yolk with the germinal vesicle or primitive ovum, is in both enclosed by a more or less clear and condensed external portion of the primitive yolk substance.

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