Book - Quain's Embryology 1

From Embryology

The Ovum

Its Maturation Fecundation and Segmentation - Formation of the Blastoderm

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Sharpey W. Thomson A. and Schafer E.A. Quain's Elements of Anatomy. (1878) William Wood and Co., New York.

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1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital
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The mature ovarian ovum

The unfecundated ovarian ovum which is approaching maturity, and is about to be discharged from the Graafian follicle and pass into the Fallopian tube, is composed of the following parts, viz. :

  1. The firm, almost homogeneous external vitelline membrane, which is termed zona pellucida in mammals
  2. The yolk substance or vitellus, a mass of soft or semifluid protoplasmic matter, involving numerous granules and oil globules, and containing embedded in it, near the surface
  3. The germinal vesicle, consisting of a spheroidal delicate enclosing membrane with protoplasmic fluid and fine granules within, and containing in its earlier states if not to the last
  4. The germinal spot or macula.

The ovarian ovum, therefore, may be regarded as a complete organised cell, in which the yolk forms the protoplasmic contents, the germinal vesicle the nucleus, and the macula the nucleolus.

Distinction of the germ

In the ovum of the mammal the whole yolk-substance is so uniformly of the same appearance and structure throughout that, but for the presence of the germinal vesicle, little distinction can be perceived between one part and another ; and, further, the greater part of the yolk is immediately engaged in the first or preliminary changes which precede the occurrence of embryonic development. The whole yolk-mass, or its greater part, is therefore directly formative or germinal, or, as it has been said, the ovum is hoJoblastic.

But in birds and reptiles, in which the ova are comparatively large, the greater part of the yolk forming the yellow yolk substance, takes no immediate part in the first formative processes, and these are restricted to the small whitish flat disc, called, the cicatricula in the fowl's egg, which is composed of fine granular protoplasm, occupies a determinate place on the surface of the larger yolk-mass, and, so long as the yolk remains in the ovary, has the germinal vesicle situated in its centre.

Fig. 489. Ovarian Ovum of a Mammifer.

a, The entire ovum, viewed under pressure ; the granular cells have been removed from the outer surface, the germinal vesicle is seen in the yolk substance within ; h, the external coat or zona burst by increased pressure, the yolk protoplasm and the germinal vesicle having escaped from within ; c, germinal vesicle more freed from the yolk substance. In all of them the macula is to the part thus distinguishable from the rest of the yolk the name of germ may be given ; and it has also been styled the primary or germinal or formative yolk, and the protoplasm or protoblast, while the remainder of the yolk-substance has been called the nutritive or food yoUc, the secondary yolk or deutoplasm. The oviparous ovum has therefore been named merodlasfic, or partially germinal.

It is not known whether in the mammals' ovum the whole yolk ought to be considered as purely germinal, or whether, as seems more probable, some nutritive yolk may not be combined intimately with the germinal substance ; but even if so, it is obvious that the germinal bears a much larger proportion to the nutritive yolk than in the bird or reptile, and, as will appear more clearly hereafter, there is thus some foundation for the distinction between the holoblastic and the meroblastic forms of ova, although it may be that in different animals these forms pass insensibly into one another.

In both kinds of ova, however, whether holoblastic or meroblastic, the subsequent phenomena of development show that the spot where the organising process begins, occupies a determinate situation in the ovum, and that the first rudiments of the embryo arrange themselves in a determinate order round a central point in the germ.

There is, therefore, in the ova of birds and mammals, a part of the yolk which is more immediately germinal, and a central point of that germ from which development spreads, to which the name of terminal pole may be given. The centre of the germ is probably coincident with the place last occupied by the germinal vesicle.

Disappearance of the germinal vesicle

The most marked change in the interior of the ovum which is known to accompany its maturation and escape from the ovary is the disappearance of the germinal vesicle, — a phenomenon which occurs in all vertebrates, and in a large proportion of, but probably not all invertebrate animals. This change is iindependent of fecundation. The details of the process have not been traced in mammals, but from various observations in birds and batrachia, and more especially from the recent minute researches of Oellacher in fishes, it follows that the disappearance of the vesicle really depends on its extrusion from the substance of the yolk in which it was imbedded, and is attended with the bursting or breaking down of its delicate outer membrane ; so that when the vesicle is thrust out on the surface of the yolk, and opened out, its fluid contents must be effused in the space intervening between the vitelline membrane and the surface of the yolk. The actual expulsion of the vesicle in the trout's ovum is attributed by Oellacher to the contractions of the yolk protoplasm, and the expulsion of the vesicle in this animal takes place previous to the rupture of its membrane and dispersion of its contents. (Archiv. f. Mikroscop. Anat. vol. viii, p. 24.)

Fig. 490. Mature Ovarian Ovum of the guinea pig (from Bischoff).

The zona pellicida is hidden by the adherent cells of the membrana granulosa, which have assumed a pediculated form next its surface. The finely granular yolk substance fills the cavity of the zona. The germinal vesicle has disappeared.

The time at which the disappearance takes place seems to be subject to some variation. Most frequently it is close upon the time of the escape of the ovum from the Graafian follicle ; but sometimes it is several hours later, and in other instances it seems to occur previously ; and, indeed, in many cases preparatory changes in the position, form, and consistence of the vesicle have been observed while the ovum was still within the ovary.

As the mammiferous ovum leaves the ovary it has still adhering to its outer surface one or two layers of the cells belonging to that part of the tunica granulosa with which it was surrounded in the Graafian follicle. These cells assume towards the period of maturation more or less of a pediculated form (see fig. 490), but after one or two days they gradually fall away from the surface of the zona, and leave that membrane free in the Fallopian tube.


Should the ovum not be fecundated it is carried down through the female passages by the ciliary action of the lining membrane, and is lost by absorption or removal. But if seminal matter is present in the tubes, and the ovum is subjected to its influence within a due time, so that fecundation is effected, there immediately follows the commencement of a series of changes in the yolk protoplasm, which result in the formation in a determinate situation of a stratum of organised cells constituting the laminar germ named the hiastoderm, which is the seat of all subsequent processes of development in the ovum.

The encounter of the ovum with the seminal filaments or spermatozoa generally takes place in the upper part of the Fallopian tube or oviduct, and it is now ascertained that the spermatozoa not only adhere in numbers to the external surface of the ovum, but actually penetrate through the zona, so as to come in contact with and possibly also combine with the substance of the yolk (see figs. 491, 492, and 493). We are, however, entirely ignorant of the nature of the operation of the spermatozoa upon the substance of the germ. The shrivelled remains of these particles are seen for days adherent to the ova, and even in the substance of the germ, and though doubtless they at last disappear, it has not been determined whether this is by combination of their substance with that of the germ or in what other way the mutual or reciprocal action of the male and female generative elements may take place.

The fact remains as one of the most remarkable in the whole range of biological phenomena, that by the contact of an inappreciable amount of the male product with the germinal material of the ovum, the latter passes from an apparently inert condition into one of genetic activity, the ultimate result of which is the accomplishment of a series of the most complicated phenomena of organic formation and growth, giving rise to a new being, which, while it may be of either sex, repeats in all respects the characters of the species, and may inherit in a greater or less degree the minutest peculiarities, whether structural or functional, of either or of both its parents.

Fig. 491. Ovum of the Rabbit from the Fallopian Tube, twelve hours after Impregnation (from Bischoff).

A few granular cells adhere to the outer surface of the zona, in which and in the zona itself spermatozoa are seen ; a, zona ; b, two hyaline globules witliin the cavity left by the shrinking of the yolk.

There are two changes following impregnation which have been observed in the mammars ovum, and which are deserving of notice, though their import is not yet known. One of these changes consists in a certain contraction or diminution in the size, and an increase in the apparent compactness or firmness of the mass of the yolk, so that a larger space than before, filled with clear fluid, comes to intervene between the yolk and the surrounding zona. The other change referred to is the appearance in this space of one, or most frequently two. and occasionally of three, clear or hyaline spherules, which are easily distinguished from the surrounding fluid by their peculiar highly refracting outline (Quatrefages, Ed. Van Beneden, Bischoff). These spherules are of variable size, but generally their diameter is from one-tenth to one-fifteenth of that of the mammal's ovum (fig. 4yi, />, and 492. A). They are perfectly hyaline and homogeneous and do not appear to possess any external envelope. They remata visible for some days during the early phases of yolk-segmentation, about to be described, and hence by some they have been named segimentation globules. Their source and destination, however, are entirely unknown.

Segmentation of the yolk or germ

After the disappearance of the germinal vesicle the germinal part of the yolk constitutes for a time a non-nucleated mass of protoplasm ; and if then subjected to the influence of fecundation it undergoes the change of segmentation, which results in the conversion of the germ or germinal part of the yolk into a layer of organised cells. This new organised structure, the blastoderm of Pander, is the future seat of embryonic development.

A segmenting process of this kind is universal throughout animals as a prelude to the commencement of embryonic development ; but it differs greatly in its extent, and somewhat also in its nature, according to the proportional relation of the directly germinal to the nutritive components of the yolk in different classes of animals. Thus in mammals, the process of cleavage appears to be complete, or to involve the whole mass of the yolk protoplasm, with which the germ is coextensive, at least in the first steps of the process; while in the bird's egg, which is pre-eminently merohlastic, the segmentation is restricted in the first instance to the disc of the cicatricula, and the great mass of the yolk substance takes no share in the change.

In intermediate forms of ova, as in amphibia and osseous fishes the segmenting division extends to a greater or less width over the yolk just in proportion to the respective limits of the germinal and nutritive parts of the yolk ; but always affecting first the germinal part, and extending subsequently outwards from the germinal pole as a centre.

Fig. 492. Ova of the Rabbit undergoing Segmentation in their descent through the Fallopian Tube. (From Bischoff.) A, the ovum from the middle part of the tube twelve or fifteen hours after impregnation, the germinal vesicle has disappeared, the yolk is contracted, and two hyaline globules are seen in the cavity between it and the zona ; rotation of the yolk took place in the direction of the arrows ; B, ovum a little more advanced, the first segmentation has taken place, a clear globule or nucleus is seen in both the yolk spheres : spermatozoa adhere everywhere to the zona ; C, an ovum four hours later than that shown in B, the second segmentation has taken place ; D, ovum from the lower part of the tube in which the third stage of segmentation is completed, and eight yolk spheres are formed, the albuminous covering is increased in thickness : diameter of the whole, 1/100th of an inch.

The process of segmentation has not been seen in the human subject, for the human ovum has not yet been detected in the progress of its descent through the Fallopian tubes ; but the phenomena have been observed with care by Bischoff and others in a variety of maramiferous animals, and as no important differences have been found to occur among them, there is no reason to doubt the similarity of the process in man. The yolk cleavage sets in within a few hours of the entrance of the mammiferous ovum into the tube, and continues to progress regularly during its descent towards the uterus, soon after its arrival in which the process is completed. The duration of this varies in different animals, being not more than from three to four days in the rabbit, in which it is the shortest known, and extending to from seven to eight days in the dog. It probably occupies not less than eight days in the human subject.

In the bird's egg the segmentation of the cicatricula is accomplished between the time of the entrance of the yolk into the oviduct and that of its being laid with its albuminous, membranous, and shell coverings, which may vary from IG or 20 to 24 or 30 hours ; and there may be some difference in the degree of completeness of the segmenting process at the time of the exclusion of the egg, according to the time the egg has taken to pass through the oviduct, the season of the year, and other circumstances.

Fig. 493. Ovum of the Rabbit sixty-eight hours after impregnation (Allen Thomson). This ovum is probably in the sixth stage of segmentation. Spermatozoa were observed within the zona, z, the zona ; a, the thick layer of albumen peculiar to the rabbit's ovum at this stage.

Segmentation of the mammals ovum

This process may be shortly described as follows : — First the whole mass of yolk protoplasm, contracted as before mentioned, splits into two somewhat ovoid or ellipsoid masses, by the formation of a fissure which begins on the surface and speedily runs through the whole thickness of the yolk (fig. 492, b). The two masses so formed lie somewhat pressed together within the vitelline membrane ; each mass presenting nearly the same appearance and structure as the whole yolk did previous to its cleavage. But as soon as this change has taken place, and according to some observers even previous to its commencement, there may be seen within each mass a small clear space similar to a nucleus. To these precursors of nuclei the name of hiasfidc has been given.

In the next stage each of the two first segments becomes cleft so as now to form four (c), each one of these having its clear spherule or nucleus within ; a third division resolves the masses into eight, of like composition with those which preceded them (d), the segments becoming of less and less size in successive stages, as meanwhile the bulk of the ovum as a whole undergoes little increase. The fourth stage ends in a division into 16 segments, the fifth into 32, the sixth into G4 (hg. 493), the seventh into 128, and the eighth into 256 (fig. 494). But it is right here to observe that while it is possible in the earlier stages to trace the reduplication of individual masses so that the succession of their numbers, when the division is complete in each stage, follows in the series of the multiples of two, yet, as the division of the different masses in any stage is not simultaneous, other and as they might be termed irregular munbers may be observed, especially in the earlier intervals of division ; as for example, three between the first and second stage, or five, six, or seven masses between the second and third, and so on. In the more advanced stages, from the great increase in number, it becomes almost impossible to follow the division of individual masses.

It is also deserving of notice that while the earlier clefts seem to pass right through the yolk and its first segments, so as to involve in the first four or five stages the whole mass of the yolk, in the later stages they do not do so, and the process seems to be in so far different, that the segmenting spheres come to be collected on the surface, and a mass of unsegmented granular and semi-fluid protoplasm or yolk substance remains within. However this may be effected, it is certain that the later division involves only the superficial set of spherules, and when the process is completed, the yolk mass comes thus to be covered by a layer of these protoplasmic spherules or segment globules, each of which possesses a nucleus and may after a time also acquire an external envelope, so as to present in all respects the features of a fully formed organised cell. (See the account of the histological relations of these spherules in the General Anatomy, p. 9).

Fig. 494. Ovum op the Rabbit from the Uterus. (from Kolliker after Bisclioff).

The whole surface of the yolk is now divided into cellular compartments. A dark spot below marks the position of a quantity of granular spheres inside the cellular elements of the blastoderm, a, the albuminous layer, now much thinned out and incorporated with the zona ; b, the cells of the outer layer of the blastoderm resulting from segmentation ; o, the spot of granular opaque spheres.

Segmentation in the bird's ovum

In the ova of birds the segmenting process is somewhat different from that now described in mammals, seeing that it is restricted to the germinal disc or cicatricuui. From the researches of Coste and several concurrent observations it appears that the first division is effected by a groove or fissure which passes through the thickness of the germinal disc, having probably a direction at right angles to the long axis of the egg. This is crossed by a second fissure so as to divide the disc into four parts near the centre. A third cleavage or fissuring is still of the same radial character, dividing the disc into eight parts or sections ; but this is succeeded by another in a different direction, which may be named concentric, and which has the effect of separating from the rest those parts of the radial segments of the disc which are next to the germinal centre : a subsequent alternating succession of radial and concentric fissures ends by dividing the whole disc into organised nucleated cells of a similar kind with those by which the whole of the mammiferous yolk becomes covered. A third set of fissures, which may be termed horizontal, must also occur to complete the separation of the segmented masses from the subjacent material. In this manner the germinal disc or cicatricula of the bird's egg has already, before the commencement of incubation, that is, during its descent through the oviduct and previous to being laid, been converted by the organising process of segmentation into the layer of cells which constitutes the blastoderm. The cicatricula of the laid egg is therefore of quite a different structure from that of the ovarian ovum, though occupying the same place and presenting much the same appearance to the unassisted eye. This layer appears to be double from the first in the bird's egg, or to consist of two strata of cells, differing somewhat in their character.

Fig. 495. Cicatricula of the Bird's Egg. (Cicatricula - the germinating or formative point in the yolk of an egg)

A, diagrammatic section tlirough the cicatricula of a newly laid egg ; a, vitelline membrane ; b, segmented germ disc ; c, below this the germ cavity ; d, the yolk cavity within the white yolk ; c, c, the yellow yolk substance.

B, view from above of the cicatricular or germ disc of a newly laid impreguatetl egg in which segmentation has been complete. The opaque area is seen surrounding the central transparent area.

C, cicatricula of an unimpregnated hen's egg, showing the vacuolar .structure produced by incomplete segmentation.

In this process there is much which is obscure and still imperfectly known, and much to excite our curiosity. The source of the first segment nucleus has not been discovered, nor is it known whether or in what way it may be related to the dispersed contents of the germinal vesicle or to its macula, and we are equally in the dark as to what may be the influence of the spermatic element upon the germ, and what the forces by which the cleavage and the formation of the multiplying spheres are brought about.

Contractile and other movements in the germ

With respect to the last mentioned topic it may be remarked that certain heaving and rotatory motions which have been observed by several embryologists immediately before and during the occui-rence of the cleavage, indicate the play of contractile and it may be of other forces within the protoplasm ; and these forces have been supposed to have some relation to the nucleus. Recent observations by Flemming in the ovum of Anodonta, of Oellacher in that of the trout, and of Goette in the toad, seem to show that there is some structural condition related to the process of division which may have a connection with its occurence ; for in the eggs of these animals the space within the domain of each segment sphere about to be formed is occupied by fine filaments radiating from the centre towards the circumference, and preceding the formation of the clear nuclear space within. It is probable that the hyaline globules may be the result of the first yolk contraction.

Secondary Segmentation

The segmenting process previously described may be called for it is not yet ascertained in how far the whole of the blastoderm, considered as the organised substratum for the development of the new being, owes its origin directly to the first process of germ segmentation, or to what extent a later process of an analogous kind may contribute to the formation of some of its deeper elements. The most recent observations, such as those of Oellacher and Goette on the egg of the bird, of Ray Lankester on the ova of Cephalopoda, and of Balfour on that of sharks, would tend to support the view that in meroblastic ova at least, the process of segmentation, considered as one of conversion of the yolk into blastodermic elements, is not completed in the first series of such divisions, but continues to take place in a modified form for some time afterwards, thus extending the blastoderm over the surface of the yolk more and more by the addition of newly acquired elements. These elements appear to be formed from nev.^ centres of cell organisation external to the limits of the germinal part of the ovum, by what may be called a process of free cell formation, and to contribute mainly to the production of the deeper part of the blastoderm. (Ray Lankester in Ann. & Mag. of Nat. Hist., 1873, p. 81, and F.M. Balfour in Jom-n. of Microscop. Science, July, 1873 and 1874 ; Goette in Ai-chiv, fiir Mikroskop. Anat., vol. x., 1874.)

Partial segmentation in unfecundated ova

It is proper further to state that although the process of segmentation as now described is the necessary preliminary to the formation of the blastoderm and is only complete in ova which have been perfectly fecundated, yet an imperfect or partial kind of segmentation has been found also to occur in unfecundated ova. This has now been observed in a variety of animals, such as mollusca, fishes, batrachia. and also in the mammiferous ovum (see Bischofi:, Ann. d. Sc. Nat. 1844. and Miiller's Archiv. 1847 : Leuckart, article '• Zeugung " in Wagner"s Handwoiterbuch der Physiol., 1852). Oellacher has recently investigated these phenomena with care in the egg of the fowl, from which it appears certain that some degree of segmentation of the germ does occur in unfecundated ova, but that it is of an irregular and incomplete kind as compared with that which follows impregnation, that it never goes on to the formation of a complete cellular blastoderm, and that although some of the earlier stages of segmentation are gone through and the g-erm is to some extent divided into segment areas, yet these are afterwards broken up by vacuoles and other unnatural processes of development, and no true blastodei-mic layer of cells is fonned (fig. 49.5, c). Enough, however, has been seen to show that some formative power resides in the germinal part of the yolk independently of the concurrence of the male element. It is not improbable that this segmentation in unfecundated ova may occur to a greater extent in the lower than in the higher animals.

1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital

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