This suggestion fell to the ground because it was held by embryologists that the cleavage of the ovumresulted in the formation of completely separate cells, and that the connexions between the adult cells were secondary.
The nucleus of the male gamete is not called the male pronucleus, as would be expected, that term being reserved for the second nucleus which appears in the ovum after fertilization.
So the process continues for some time until the ovum becomes possessed of a large number of nuclei, all of which have proceeded from the original nucleus by a series of binary fissions.
It was at this time that von Baer made the important discovery of the ovarian ovum of mammals and of man, totally unknown before his time, and was thus able to prove as matter of exact observation what had only been surmised previously, viz.
The food-yolk is deposited in the ovum as a result of the vital activity of its protoplasm, while the ovum is still a part of the ovary of the parent.
In the cleavage of the ovum the first two of these methods of division are found, but probably not the third.
It is characteristic of this kind of ovum that there is always a row of nuclei, called the yolk-nuclei, placed in the denser yolk immediately adjacent to the blastoderm.
Peripatus Capensis the cells of the segmenting ovum do not separate from one another, but remain connected by a loose protoplasmic network.
Sometimes the ovum arises from the oocyte by one division only, and there is only one polar body (e.
The oocyte (progamete) gives rise by division to the ovum or true gamete, the nucleus of which is called the female pronucleus.
Should an ovum escape into the uterus, it may become impregnated a month or so after a previous conception.
Several days may elapse before the spermatozoa meet with an ovum and fertilize it.
In very rare cases, the ovum has been observed to undergo a certain amount of development of itself; but a perfect individual can be produced only by the union of the two kinds of elements, which process is known as fecundation.
Reproduction is effected through the union of the ovum with the zoosperm.
Sometimes the ovum becomes fecundated before reaching the uterus, and, instead of passing onward into that organ as usual, remains in its position in the Fallopian tube or even on the surface of the ovary.
As a general rule, in the human female, a singleovum is developed and discharged once in about four weeks, during the period of sexual activity.
This theory is strikingly confirmed by the fact, previously mentioned, that in certain cases the ovum alone, a single reproductive element, may undergo a degree of development approaching very near to completion.
It is connected with a duct which receives the ovumas it is discharged, and conveys it to the uterus.
When an ovum is matured, it escapes from the ovary into the narrow tube referred to, called the Fallopian tube, and passes down into the cavity of the uterus.
It has been quite satisfactorily settled that the discharge of the ovumfrom the ovary generally takes place about the time of the cessation of the flow.
Occasionally an ovumfalls into the cavity of the abdomen instead of passing into the tube.
When an ovum escapes from the ovary in connection with menstruation, it is by these delicate hairs propelled along a filament of tissue to the Fallopian tube, and thence by the same means is conveyed to the uterus.
As soon as theovum is impregnated by the male element, it begins a process of symmetrical division.
The gauze can be removed in from twelve to twenty-four hours, and the ovum will generally be found lying upon the upper part of the packing, or in the canal that is now dilated, from which it can easily be removed.
Then follows the development of this ovum and with it the development of the womb, and this growth or development constitutes the process which is called pregnancy.
The folds by which it is surrounded then grow forward and their edges unite over the egg or ovum forming a sac--the decidua reflexa.
After the ovum has been expelled an antiseptic vaginal douche should be given twice a day for a week or longer.
If in the course of its travel from the ovaries, through the tube to the womb, the female ovum or egg meets with the male elements, fertilization or impregnation may take place.
Children from the same ovum (egg) are always of the same sex.
Let us recall in outline the early history of the fertilized germ-cell, the zygote formed by the union of ovum and spermatozoön.
If, however, the ovum is not impregnated, because of absence of copulation or from other causes, it gradually loses its vitality, and is finally cast off with the several uterine secretions.
Any such ovum contains elements of constitution from both of its parents.
But the ovum that is successful finally arrives at the Uterus where it awaits impregnation or fertilization by the spermatozoon of the male.
Fecundation results and conception ensues, the ovum then remaining attached to the walls of the Uterus, and in time develops into the foetus.
The moment that the real body of the ovum is penetrated by the successful spermatozoon, a tough covering or thick membrane forms around the ovum and thus prevents the entrance of other spermatozoa.
In woman one ovum is usually discharged each month, and it is maintained that on one month the ovum is male-determining, and in the next, female-determining.
There now appears at one point on the ovum an opaque streak, which is called "the primitive trace" of the embryo--the first beginning of the young living creature.
But wherever the actual contact of spermatozoon and ovum occurs, the blending of the elements is performed and fertilization, impregnation, or fecundation is accomplished.
A thick, spongy, juicy, mucus membrane forms, into which the changing ovumpasses and attaches itself; the mucus membrane soon enveloping it and shutting it off from the rest of the Uterus.
To begin with, there is in the ovum and early embryo no nervous system at all, and it develops very gradually from simple beginnings.
Darwin called the pinhead brain of the ant the most marvellous atom of matter in the world, but the human ovum is more marvellous still.
The thumb is then replaced over the end of the tube, which is lifted from the water with the ovum retained in it.
They generally come out tail first, and in wriggling this about in their attempts to get further out, they propel the ovum about the bottom of the tray.
Now, as an ovum is specially developed every month, it follows that this ovum requires a certain part of a month to attain a more or less advanced degree of ripeness.
If the activity of the generative apparatus of the female should be increased by any circumstance, in the case of the animals the ripening of the ovum would be accelerated.
When, during the rutting period, an ovum is detached from the ovary, and passes through the Fallopian tubes to the cavity of the uterus, the fructification can take place at the beginning of the rutting period.
But when the fecundation has taken place at the end of the rutting period, the ovum has reached its highest degree of development, and, if effectively fecundated, it will become a male.
And so I again come back to this, that we may expect a male individual from the ovum when the juices are developed which serve as a functional means of impulse for the male sex.
He does not appear to be disposed to admit the existence of male and female ova in the ovary; but he thinks it absolutely certain that the sex is determined at conception by the reciprocal action of the ovum and the semen.
In the growth of the fertilized ovum we have to deal again with a phenomenon of impulse, a part of which is the property of the ovum itself, a part, however, also dependent upon external influences.
As we had exercised no influence upon it so as to effect any change in the ovum (in the same way as we had previously been able to affect the others by the diet in order to procure the ovum of a male) the result was a female.
The assumption of a greater or lesser degree of ripeness of theovum which was to be developed was a very questionable one.
That ripeness of the ovum upon which Thury's theory insists, lies in the nature of the ovum apart from any anatomical signs.
On the impregnation of the Ovum in Amphibia" (three memoirs).
In accordance with this law the cells of the Amphibian ovum are of unequal size even at the close of segmentation.
In many cases peculiar processes or villi grow out from the ovum (fig.
Section through part of the ovum during segmentation.
The occurrence of a meroblastic segmentation, in spite of the ovum being usually smaller than that of Amphibia and Acipenser, etc.
Contributions to the history of the structure of the human ovum and embryo before the third week after conception; with a description of some early ova.
Front and side view of an ovum figured by Reichert, supposed to be about thirteen days.
The lower pole of the ovum is then formed of a mass in which no traces of segments or segmentation furrows can be detected.
Owing to the development of food-yolk in the Elasmobranch ovum the early stages of development are to some extent abbreviated, and almost all trace of a stage with a suctorial mouth has become lost.
Each of the polar bodies removes maternal chromatin from the ovum to make room for the chromatin of the fertilizing spermatozoön; but their functions are not fully understood.
A term collectively applied to the changes or conditions preceding fecundation, especially to the changes which the ovumundergoes before fecundation.
The chance of an ovum being placed in a situation where it will be hatched, and the young find convenient subsistence, must be very small; hence the necessity for their being very prolific.
The science which relates to the formation and development of the embryo in animals and plants; a study of the gradual development of the ovum until it reaches the adult stage.
Formation and separation of laminæ or layers; one of the methods by which the various blastodermic layers of the ovum are differentiated.
Characterizing the ovum when it has two primary germinal layers.
The lifeless food matter in the cytoplasm of an ovum or a cell, as distinguished from the active or true protoplasm; yolk substance; yolk.
Within these, instead of the single ovum may be seen a swarm of sperms, minute and highly active.
In considering the union of ovumand sperm as a source of variation, we have taken for granted the existence of variations.
While in cases of alternation of generations, "an entire asexual generation, or more than one, may intervene between one ovum and another.
The ovum on this view is to be regarded as a composite germ containing the germs of the cellular constituents of the future organism.
In the case of the bird, the protoplasm of the ovum has acquired the power of storing up the by-products of its vital activity.
G] On the other hand, three ova of the crustacean Apus are said to coalesce to form the single ovum from which one embryo develops.
Here, again, we notice that one fertilized ovum gives rise to not one, but a number of liver-flukes.
I believe that it brings an inherited tendency to fission, and thus reintroduces into the fertilized ovum the tendency which, as ovum, it had renounced in favour of storage katabolism.
F] It is not, however, necessary in all cases that fertilization of the ovum should take place.
The sperm is also a nucleated cell; and on its entrance into the ovum there are for a short time two nuclei--the female nucleus proper to the ovum, and the male nucleus introduced by the sperm.
In a few types the polar cells are formed while the ovum is still in the ovary, as, for instance, in some species of Echini, Hydra, etc.
It is secreted by the protoplasm of the ovum, and has a gelatinous consistency, and after it is formed the peripheral layer of the protoplasm of the ovum sends out through it pseudopodia-like processes to absorb nutriment from without.
The capsule of the ovum then bursts at the opposite pole to the peduncle, and the ovum falls into the oviduct.
After the growth of the ovum is completed the amoeboid processes gradually withdraw themselves, and the ovum assumes a spherical form; still however continuing to be invested by the remaining cells of the ovary.
At the time of impregnation the ovum is a naked cell, and undergoes in this condition the earlier phases of segmentation.
Just beyond the edge of the ovum the small cells have begun to undergo retrogressive changes; while at a little distance from the ovum they are quite normal (g.
The mode of nutrition of the ovum may be very instructively studied in this type.
The ovum is dehisced into the body cavity by the bursting of its capsule or the rupture of the stalk.
Its change of position in a large number of instances is accomplished during the growth of the ovum in the ovary, but in other cases does not take place till the ovum has been laid.
In some cases the ovum while still retaining the constitution last described becomes invested by a very delicate membrane.
The ovum now becomes more or less separate from the mass of germinal cells, rapidly grows in size, and soon forms the most considerable constituent of the follicle (fig.
Now, if we study the characteristics of the ovum and of the spermatozoA¶n we may be surprised when we discover some of the same differences which characterize men and women from infancy to old age.
Now, the human ovum or egg consists of a vitellus or yolk, which is covered by a similar membrane, known as the vitelline.
That one month the ovum is of that character which would result in the production of a male child, and the succeeding month of such a nature as would result in the production of a female child.
God might just as easily have ordained that the ovum should be complete in itself, and that, without any intervention or co-operation, at appointed intervals the mother should bring forth her offspring.
The ovum may remain for a period, and the spermatozoa that seek the ovum may enter, but neither may tarry unless each yields itself to the other in that mysterious union which results in life.
In the highest forms of life the ovum reaches its maturity in a department of the mother's body which is called the ovary.
While the ovum is passive, the sperm of the male is characterized by great activity and remarkable vitality.
When theovum is quickened, every fibre of being thrills with a new purpose.
While, of course, the greatest changes which take place during the period of gestation are chiefly within the ovum or egg, yet those which take place in the body of the mother herself are more manifest, and scarcely less marvelous.
From the moment that the spermatozoA¶n penetrates the ovum and unites with it, life is present, and the destruction of that life is murder.
Very early miscarriages may be explained by the loose attachment of the ovumduring the first six weeks of pregnancy.
Obviously, a shell would not only be useless to an embryo developing within the body of its parent, but would shut off the nourishment, which, since the ovum contains so little, must necessarily be provided by the mother.
The primary trunks with all their branches hang from the capsule of the ovum and extract nutriment from the mother's blood which surrounds them, just as the roots of a tree extract it from the soil.
The expulsion of the after-birth frees the mother of all the tissue derived from the growth of the ovum, for the intricate mechanism that served to nourish and protect the embryo was almost entirely developed from the ovum itself.
Similarly we might expect the ovum to be the bearer of the maternal qualities inherited by the child.
The material which conveys inheritable characters can be seen and has been identified in both germinal cells; from each of them the fertilized ovumderives equal amounts.
Thither a number of the male cells have traveled by their own activity; several come in contact with the ovum and one, but only one, actually enters it.
By this means it has been definitely proved that the ovum multiplies rapidly after it has been fertilized, and becomes, as was explained in the preceding chapter, a sac-like structure within which hangs a tiny clump of tissue.
At first, however, its roots are short and delicate, and not so capable of anchoring the ovum as they become later.
Shortly after leaving the ovary the ovummust receive the stimulus to live and grow; otherwise it will quickly wither and die.
Pregnancy modifies every portion of the womb in one way or another; but the most profound alterations occur in the body, in the cavity of which the ovum has come to rest.
But just before implantation takes place the architecture of the ovum is modified in such a way as to indicate the trend of its subsequent development.
Although every ovum that will be produced as long as a woman lives has already sprung into existence by the time she is born, not a single one ripens for from twelve to fifteen years.
By this means the ovum receives a rich supply of nutriment, and as a natural consequence its growth is rapid.
In most animals it is definitely known that the sex of the offspring has been fixed when the male cell enters the female cell, in other words, at the instant the ovum is fertilized.
This ovum is extremely small, being a tiny round vesicle about the 1/120 of an inch in diameter; it can be seen under very favourable circumstances with the naked eye as a tiny particle, but is otherwise quite invisible.
In these cases the cleavage of the ovum begins at the upper pole, and leads to the formation of a dorsal discoid embryo.
Both the segmentation of the ovum and the subsequent gastrulation have in this way been considerably changed.
This membrane is the follicle, from which the ovum afterwards issues.
Only a small part of the surface of the ovum is shown.
Here, also, the ovum is originally of the same character as it is in the mammal; it is a perfectly simple and naked cell.
The impregnation of the ovum commences with a decay of the germinal vesicle, or the original nucleus of the ovum (Figure 1.
We then see that the original ovum is a quite small, naked, and simple cell with a nucleus, not differing in either size or shape from the original ovum of the mammals and other animals (cf.
When the mature bird-ovum has left the ovary and been fertilised in the oviduct, it covers itself with various membranes which are secreted from the wall of the oviduct.
The cleavage of the frog's ovum (magnified ten times).
We know that the paternal half of the reproductive elements does not enter the ovum till a comparatively late stage in its history, and it is quite possible that maternal elements or gemmules may also enter the ovum from without.
This is the first attempt of the fertilised ovum to form itself into the different layers, which are ultimately going to give rise to all the different tissues of the embryo.
The uterine wall, after this embedding of the ovum within it, undergoes a remarkable growth at this position, concerning which a word must be said.
They cannot develop independently, but when they unite with the egg or sperm of another individual, a new fertilised ovum is formed and the cycle begins again.
In other words there has never been any conjugation between one fertilised ovum and another in spite of the generations of cells which have been produced between them.
Unless fertilisation takes place by union with a sperm the discharged ovum perishes.
The lining membrane of the uterus under ordinary conditions measures about one-eighth of an inch in thickness, but, after the ovum has become embedded in it, it begins to increase until it reaches as much as half an inch.
This process being carried still further allows the ovum to sink deeper and deeper into the lining membrane of the uterus.
But before fertilisation can take place the ovum has undergone what is called the process of maturation, in which it divides twice, giving off two small portions of itself in the process.
Each successive step from the very beginning of the fertilisation of the ovum repeats a stage through which previous generations have passed.
The cell contributed by the female, the ovum that is, has quite a different structure and microscopical appearance.
The embryonic area is the only part of the ovum which has to do with the subsequent development of the embryo; the other parts of the blastodermic vesicle become subservient as nutritive or supporting structures.
These two cells in their turn were also derived in a straight line of descent from the fertilised ovum from which each parent sprang.
This means that the zygote or fertile ovum contains double the number of chromosomes possessed by either gamete, and that, moreover, each tissue-cell of the new individual will contain this dual number.
And it is a very significant fact that while the ovum and spermatozoon are very unequal in size themselves, the chromosomes of the two germ-cells are of the same size and number.
Or such a cell may combine with an ovum carrying the defective X, also producing a female but one who although of normal color vision herself, like her mother, is a carrier of the defect.
As regards the behavior of the chromosomes the maturation of the ovum parallels that of the sperm-cell.
A sex-cell carrying normal X of the male may combine with an ovum containing normal X producing a normal female (row five).
Each ovum finally enters on a series of cell-divisions forming a mass of smaller and smaller cells which gradually assumes the form of a hollow sphere like the parent colony.
The fact that they are invariably of the same sex is a strong reason for believing that sex was already developed in the fertile ovum and consequently in the resulting blastomeres from that ovum.
Ultimately each ovum is penetrated by a spermatozoon, the two cells fuse completely and thus form the single fertilized ovum or zygote.
