Book - Text-Book of the Embryology of Man and Mammals 2
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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The Phenomena of the Maturation of the Egg and the Process of Fertilisation
The Phenomena of Maturation
EGGS, such as have been described in the previous chapter, are not yet capable of development, even if they have acquired the normal size. Upon the addition of mature semen they remain unfertilised. In order that they may be fertilised they must first pass through a series of changes, which I shall group together as the phenomena of maturation.
The maturation-phenomena begin with changes of the germinative vesicle, which have been followed out the most carefully on the small transparent eggs of invertebratecl animals, such as the Echinoderms and Nematodes (the maw-worm of the horse). The germinative vesicle gradually moves from the middle of the egg the egg of an Echinoderm may serve as the basis of the description towards its surface, shrivels a little (fig. 12 A), in that fluid escapes from it into the surrounding yolk, its nuclear membrane disappears, and the germinative dot becomes indistinct and breaks up into small fragments (fig. 125 &/"). During this degeneration of the germinative vesicle a nuclear spindle (fig. 12 B sp) is formed, as can be recognised only after appropriate treatment with reagents ; there arises out of parts of the germinative dot, or out of a part of the nuclear substance of the germinative vesicle, a nuclear spindle (fig. 12 .Z? sp),- a form of the nucleus which one encounters in the animal and vegetable kingdoms in stages preparatory to cell-division.
The nuclear spindle, the more precise structure of which will be described later, in discussing the process of cleavage, pursues still further the direction already taken by the germinative vesicle, unti it touches with its apex the surface of the yolk, where it assumes a position with its long axis in the direction of a radius (fig. 137 sp). A genuine process of cell-division soon takes place here, which is to be distinguished from the ordinary cell-division only by this, that the two products of the di vision are of very unequal size. To be more exact, therefore, we have to do here with a cell-budding. At the place where the nuclear spindle touches the surface with one of its extremities the yolk arches up into a small knob, into which half of the spindle itself advances (fig. 13/7). The knob thereupon becomes constricted at its base, and with the half of the spindle from which subsequently a vesicular nucleus is again formed is detached from the yolk as a very small cell (fig. 13 777 rk l ). Hereupon exactly the same process is repeated, after the half of the spindle which remains in the egg, without having previously entered into the vesicular quiescent stage of the nucleus, has restored itself to a complete spindle (fig. 13 IV).
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Fig. 12. Portions of eggs of Asterias glacialis, They show the degeneration of the germinative vesicle.
In figure A it begins to shrivel, in that a protuberance of protoplasm (x), with a radial structure inside of it, penetrates into its interior, and dissolves the membrane at that point. The genuinative dot (kf) is still visible, but separated into two substances, nuclein (/i) and paranuclein Gm).
In figure B the germinative vesicle (kV) is entirely shrivelled, its membrane is dissolved, and only small fragments of the germinative dot (kf) remain. In the region of the protoplasmic protuberance of figure A there is a nuclear spindle (.?/)) in process of formation.
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There now lie close together on the surface of the yolk two spherules, which consist of protoplasm and nucleus, and therefore have the value of small cells (fig. 13 V rk 1 , rk 2 ), and which are often to be identified in an unaltered condition, even after the egg has been divided into a number of cells. They were already known in earlier times under the name of direction bodies, or polar cells. They have acquired the latter name because, in the case of eggs in which an animal pole is to be distinguished, they always arise at that pole. After the conclusion of the second process of budding, one half of the spindle, the other half of which was employed in the formation of the second polar cell, is left in the cortical layer of the yolk (fig. 13 F and VI ek). From this arises a new, small, vesicular nucleus, which consists of a homogeneous, tolerably fluid substance without distinctly segregated nucleoli, and attains a, diameter of about 13 /x,. From the place of its formation it usually migrates slowly back again toward the middle of the egg (fig. 14 ek). The nucleus of the mature egg (fig. 14 ek) has been designated by me as Egg-nucleus, by VAN BENEDEN as female pronucleus, It is not to be confounded with the germinative vesicle of the unfertilised e<j<j. Compare the figures of the immature egg (fig. 15) and the mature egg (fig. 14) of an Echinoderm, both of which are drawn with the same magnification* The germinative vesicle is of very considerable size, the egg-nucleus remarkably small : in the case of the former one distinguishes a clearly developed nuclear membrane, a nuclear network, and a nucleolus ; the latter is almost homogeneous^ without nucleolus, and not separated from the protoplasm by any fixed membrane. Similar distinctions in the condition of the germinative vesicle and the egg-nucleus recur throughout the animal kingdom.
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Fig. 13. Formation of the polar cells in Asterias glacialis. In figure /. the polar spindle (s/j) has advanced to the sxirface of the egg. In figure //. there has been formed a small elevation (rk l ), which receives a half of the spindle. In figure ///. the elevation is constricted off, forming a polar cell (rA: 1 ). Out of the remaining half of the previous spindle a second complete spindle (.s/j) has arisen. In figure IV. there bulges forth beneath the first polar cell a second elevation, which in figure V. has become constricted off as the second polar cell (rk y ). Out of the remainder of the spindle is developed (figure VI.) the egg-micleus (ek).
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The formation of polar cells, and the accompanying metamorphosis of the germinative vesicle into such an extraordinarily reduced eggnucleus, is a phenomenon of very wide, probably, indeed, of general occurrence. Polar cells have been observed throughout the Ccelenterates, Echinoderms, Worms, and Molluscs. In the ripening of the eggs of Arthropods, according to the earlier observations, they appeared never to be present; but recently they have been found in numerous species by a number of observers, especially by BLOCHMANN and WEISMANN. Among Vertebrates polar cells are always encountered in Cyclostomes and Mammals, whereas in Fishes and Amphibia they have been identified only in some cases, and in Reptiles and Birds not at all as yet. They arise either some time before or else during fertilisation.
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Fig. 14. Mature egg of an Echinoderm. It encloses in the yolk the very small homogeneous egg-nucleus (e).
Fig. 15. Immature egg from the ovary of an Echinoderm,
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In the case of Mammals (Rabbit and Mouse) the process has been very carefully investigated by VAN BENEDEN, and recently by TAFANI. Several weeks before the rupture of the GRAAFIAN follicle the germinative vesicle ascends to the surface of the egg ; some days before that epoch it there disappears, and at the place where it disappeared there are formed the egg-nucleus and, under the zona pellucida, one or two (TAFANI) polar cells. The egg after it has escaped from the ovary always exhibits egg-nucleus and polar cells.
Also in the case of Fishes, Amphibia, Reptiles, and Birds, whose eggs are of considerable size and with few exceptions opaque, the germinative vesicle, distinguished by its numerous nucleoli, undergoes a regressive metamorphosis. As has been followed step by step in Teleosts by OELLACHER, and in Amphibia by the author, it always ascends from the middle of the yolk to its surface, and in fact without exception to its animal pole : in the case of the Frog (fig. 16 kb) this occurs many weeks before the beginning of maturation. Here immediately under the vitelline membrane, it becomes flattened to a disc-like body, being at the same time somewhat shrunken. Further changes, which it is very difficult to follow in detail, take place in a comparatively short time ; these occur in the case of the Amphibia at the time when the
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Fig. 16. Frog's egg in process of ripening, The germinative vesicle (i'6), with numerous germinative dots (A/), lies c^uite at the surface of the animal role as a flattened lenticular body.
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eggs are detached from the ovary. For if one examines eggs which have already escaped into the abdominal cavity, or have entered the oviduct, it is uniformly found that the germinative vesicle with its dots has disappeared. In this case, too, there are subsequently formed from a part of the chromatic substance of the germinative vesicle two polar cells and an egg-nucleus, as has been proved by the fine investigations of HOFFMANN for some species of Teleosts, of O. SCHULTZE for several Amphibia (Siredon, Triton), and of KASTSCHENKO for certain Selachians.
WEISMANN and BLOCHMANN have discovered a very interesting fact in the Arthropods. In eggs, namely, which develop parthenogenetically (in summer eggs of Polyphemus, Bythotrephes, Moina, Leptodora, and Daphnia, as well as in Aphidse) only a single polar cell is eliminated, whereas in eggs which require fertilisation for their further development there are always two formed. At present, however, this contrast cannot be established as a general law. For PLATNER found that in the case of Liparis dispar there are formed in parthenogenetic eggs, as well as in those which are fertilised, two polar cells, the first of which again divides. BLOCHMANN arrived at the same result from the investigation of unfertilised eggs of bees, from which drones are developed.
Although the researches on the phenomena of maturation of the egg in animals still present numerous gaps, nevertheless it can be regarded as already well-established, that eggs with a germinative vesicle are never capable of fertilisation, that the germ/motive vesicle is without exception dissolved, and that there is formed out of components of it (as regards the details there are still many processes to be more carefully studied) a very small egg -nucleus. During the metamorphosis there arise, probably without exception, polar cells.
The polar differentiation of many eggs rich in yolk, which was pointed out in the first chapter, may be brought into causal connection with the phenomena of maturation. Without exception the animal pole is the part of the egg-sphere to which the germinative vesicle ascends, and where the polar cells are subsequently formed. That the protoplasm is accumulated here in greater quantity is in part referable to the fact that it comes to the surface of the egg along with the nucleus, which most certainly furnishes a centre of attraction for the protoplasm.
The insight into the phenomena of the maturation of the egg, as they have been connectedly presented in the preceding pages, has been acquired only by many roundabout ways and after the removal of many misconceptions. As early as the year 1825 PUEKINJE, the discoverer of the germinative vesicle in the Hen's egg, found that in eggs which were taken from the oviduct this vesicle had disappeared, and from this concluded that it was ruptured by the contractions of the oviduct, and that its contents (a lympha generatrix) were mingled, with the germ. Whence the name vesicula germinativa. Similar observations were made on this and other objects by C. E. v. BAER, OELLACHER, GOETTE, KLEINENBERG, KOWALEVSKY, BEICHERT, and others. But on the other hand the positive statements were made for many eggs (by JOH. MULLER for Entoconcha mirabilis ; by LEYDIG, GEGENBAUR, and VAN BENEDEN for Eotifers, Medusas, etc.) that the germinative vesicle did not disappear, but remained and gave rise by direct division at the time of segmentation to the daughter-nuclei.
There were therefore in previous decennia two opposing parties : the one asserted the continuance of the germinative vesicle and its division during the process of cleavage ; the other maintained that the egg-cell in its development passed through a condition without nucleus, and again acquired a nucleus in consequence of fertilisation.
The controversial points were cleared up by investigations which BUTSCHLI and the author had undertaken at the same time.
I showed in my first " Beitrage zur Kenntniss cler Bildung, Befruchtung und Theilung des thierischen Eies," that in all the older writings there had been no distinction made between the nucleus of the immature, the mature, and the fertilised egg, but that these nuclei had been often confounded and held to be identical, and I first established the differences between germinative vesicle, egg-nucleus, and cleavage-nucleus, the latter being the names which were introduced by me. In addition I showed that the disappearance of the germinative vesicle and the origin of the egg-nucleus preceded fertilisation, and thus I distinguished between the phenomena of maturation and fertilisation of the egg-cell, which generally had been interchanged and confounded. I also endeavoured to make it probable that the egg-nucleus descended from the germinative vesicle, and in fact from a nucleolus of the vesicle, and defended the thesis that the egg during its maturation did not pass through a non-nuclear condition. In this I fell into an error : I overlooked, like all previous observers, the connection between the formation of the polar cells and the disappearance of the germinative vesicle, a process which it was the more difficult to establish in the object which I studied because it takes place in the ovary.
The excellent investigations of BUTSCHLI, which brought the changes of the germinative vesicle into connection with the formation of the polar cells, now made their appearance, supplementing my results. The polar cells were discovered in the year 1848 by FR. MULLER and LOVEN, and were named by the former directive vesicles (Richtungsblaschen), because they always lie at the place where subsequently the first cleavage-furrow makes its appearance. Their wide distribution in the animal kingdom had also been established by many investigators ; BUTSCHLI was the first, however, to direct attention to the peculiar processes which take place in the yolk, in the interpretation of which he, nevertheless, committed several errors. He maintained that the whole germinative vesicle is converted into a spindle-shaped nucleus, which moves to the surface, and, while becoming constricted in the middle, is thrust outside by the contractions of the yolk in the form of two directive bodies. By this process the egg became non-nuclear, and again acquired a nucleus only in consequence of fertilisation.
In two further articles on the Formation, Fertilisation, and Cleavage of the Animal-Egg, I modified the teachings of BUTSCHLI, and brought them into unison with my previous investigations, inasmuch as I pointed out that the germinative vesicle is not as such directly converted into the nuclear spindle, but in part is dissolved : that the spindle takes its origin from the nuclear substance in a manner which it is very difficult to investigate ; that the polar cells are formed, not by the elimination of the spindle, but by a genuine process of division or budding ; that in consequence of this the egg is not destitute of a nucleus even after the constricting off of the second polar cell, but that the egg-nucleus arises from the half of the divided polar spindle which remains in the yolk, and therefore, in its ultimate derivation, from components of the germinative vesicle of the immature egg.
Soon afterwards BUTSCHLI also interpreted the development of the directive bodies as cell-budding, likewise GIARD and also FOL, who has produced a very extensive and thorough investigation on the phenomena of the maturation of the egg in animals. Kecently VAN BENEDEN, supported by researches on Nematodes, has conibatted the interpretation of the process as cell-budding; however, BOVERI and 0. ZACHARIAS, who have established a complete agreement between the formation of directive bodies and the process of cell-division in the case of the Nematodes also, are unable to subscribe to his conclusion in this matter.
As a new advance is to be recorded the discovery by WEISMANN and by BLOCHMANN, that in eggs which are developed parthenogenetically only a single polar cell arises.
If the original obscurity on the morphological side, in which the phenomena of the maturation of the egg were enveloped, has been in general cleared up, the same is not the case if we inquire after its physiological meaning. That the germinative vesicle undergoes a regressive metamorphosis into component parts is easily comprehensible, for a firm membrane and a rich accumulation of nucleoplasm certainly cannot be necessary to the interaction of protoplasm and active nuclear substance in the processes of division. Its dissolution is, as it were, the preliminary requirement for the renewed activity of the nuclear contents. But what function shall one ascribe to the polar cells ? Concerning this several hypotheses have been proposed.
BALFOUR, SEDGWICK MINOT, VAN BENEDEN," and others, are of opinion that the immature egg, like every other cell, is originally hermaphroditic, and that by the development of polar cells it rids itself of the male constituents of its nucleus, which afterwards are replaced by fertilisation. BALFOUR thinks that, if no polar cells were formed, parthenogenesis must normally occur.
WEISMANN, supported by his discovery in the case of eggs developing parthenogenetically (p. 34), ascribes a different function to the first and the second polar cells. He distinguishes in the germinative vesicle two different kinds of plasma, which he designates ovogenetic and germinal plasma. He maintains that by the formation of the first polar cell the ovogenetic plasma is eliminated from the ovum ; by that of the second polar cell, half of the germinal plasma. In the latter case the ejected germinal plasma must be replaced by fertilisation.
These hypotheses appear to me upon closer examination to present many vulnerable points. To me appears more promising an interpretation of BUTSCHLI, who compares the egg, as had already often been done, to the mother-cell of spermatozoa. Just as the latter gives rise to many spermatozoa, so also the egg must have once possessed the capability of dividing itself into many eggs. In the formation of the polar cells, which are eggs that have become rudimentary, as it were, there has been preserved a trace of these original conditions. Also BOVERI regards the polar cells as abortive egg*. I have likewise always conceived of the conditions in this manner.
The Process of Fertilisation
The union of egg-cell and spermatic cell is designated as the process of fertilisation. This process is to be observed, sometimes with great difficulty, sometimes with considerable ease, according to the choice of the animal for experimentation. The investigator ordinarily encounters great difficulties in cases where the ripe eggs are not laid, but where a part, if not the whole, of their development is effected within the sexual ducts of the maternal organism. In such cases the fertilisation also must evidently take place in the ducts of the female sexual apparatus, into which the semen is introduced in the act of copulation.
An internal fertilisation takes place in nearly all Vertebrates except the greater part of the Fishes and many Amphibia. Usually the egg and the spermatozoa meet, : in the case of Man and Mammals, in the beginning of the oviduct; likewise in the case of Birds they meet in the first of the four regions previously (p. 17) distinguished, and at a time when the yolk is not yet surrounded with its albuminous envelope and calcareous shell.
In contrast to internal fertilisation stands external fertilisation, which is the simpler and more primitive method, and which occurs in the case of many Invertebrates that live in the water, as well as ordinarily in Fishes and Amphibia. In this method, while male and female keep near together, both kinds of sexual products, which are for the most part produced in great number, are evacuated directly into the water, where fertilisation takes place outside of the maternal
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Fig. 17 A, B, C. Small portions of eggs of Asterias glacialis, after FOL.
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The spermatozoa have already penetrated into the gelatinous envelope which covers the eggs. In A there begins to be raised up a protuberance toward the most advanced spermatozoon. In B the protuberance and spermatozoon have met. In C the spermatozoon has penetrated into the egg. A vitelline membrane, with a crater-like orifice, has now been distinctly formed.
organism. The whole procedure is therefore much more easily observable. The experimenter has it within his power to effect fertilisation artificially, and thus to determine precisely the point of time at which egg and semen are to meet. He needs only to collect in a watch-glass containing water ripe eggs from a female, likewise in a second watchglass ripe semen from a male, and then to mingle the two in a suitable manner. In this way artificial fertilisation is extensively practised in fish-breeding. For the purpose of scientific investigation the selection of the particular species of animal is of the greatest importance. It is manifest that animals with large opaque eggs do not commend themselves, whereas those species are especially suitable whose eggs are so small and transparent that one can observe them under the microscope with the highest powers, and at the same time pass in review every least speck. Many species of Echinoderms are in this respect most excellent objects for investigation. Consequently it was by means of them that an accurate insight into the processes of fertilisation was first secured. They may therefore serve in the following account as the foundation of our description.
If ripe eggs with egg-nucleus are removed from the ovary into a watch-glass containing sea -water, and a small quantity of seminal fluid is added, a very uniform result is obtained, since in the course of five minutes every one of many hundreds or thousands of eggs is normally fertilised, as can be accurately observed by means of high magnification.
Although spermatozoa attach themselves to the gelatinous envelope of an egg in great numbers, many thousands of them when concentrated seminal fluid is employed, still only a single one of them is concerned in fertilisation, and that is the one which by the lashlike motion of its filament first approached the egg. Where it strikes the surface of the egg with the point of its head the clear superficial expanse of the egg-protoplasm is at once elevated into a small knob that is often drawn out to a fine point, the so-called receptive prominence (Empfdngnisshifgel), or cone of attraction. At this place the seminal filament, with pendulous motions of its caudal appendage, bores its way into the egg (fig. 17 A, B). At the same time a fine membrane (fig. 71 C) detaches itself from the yolk over the whole surface, beginning at the cone, and becomes separated from it by an ever-increasing space. The space probably arises because, in consequence of fertilisation, the egg-plasma contracts and presses out fluid (probably the nuclear fluid which was diffused after the disappearance of the germinative vesicle).
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Fig. 18. Fertilised egg of a Sea-urchin. The head of the spermatozoon which penetrated has been converted into a sperm-nucleus surrounded by a protoplasmic radiation, and has approached the egg-nucleus (d-). Fig. 19. Fertilised egg of a Sea-urchin. The sperm-nucleus (sfc) and the egg-nucleus (ek) have come close to each other, and both are surrounded by a protoplasmic radiation.
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The formation of a vitelline membrane is in so far of great significance for the fertilisation, as it makes the penetration of another male element impossible. No one of the other spermatozoa swinging to and fro in the gelatinous envelope is able after that to get into the fertilised egg.
The one which has penetrated thereupon undergoes a series of changes. The contractile filament ceases to vibrate, and soon disappears ; but out of the head which, as was previously stated, is derived from the nucleus of a sperm-cell (sperm atid), and consists of nuclein there is soon developed a very small spheroidal or oval corpuscle, which afterwards becomes somewhat larger, the semen- or sperm-nucleus (fig. 18 sk). This slowly moves deeper into the yolk, whereupon it exerts an influence upon the surrounding protoplasm. For the latter is arranged radially around the sperm-nucleus (sk}, so that there is formed a radiate figure, which is at first small, but afterwards becomes more and more sharply expressed and more extended.
Now an interesting phenomenon begins to hold the attention of the observer (figs. 18, 19, 20). Eggnucleus and sperm-nucleus mutually attract each other, as it were, and migrate through the yolk toward each other with increasing velocity. The sperm-nucleus (sk), enveloped in its protoplasmic radiation, changes place more rapidly than the egg-nucleus (ek). Soon the two meet, either in, or at least near, the middle of the egg (fig. 19) ; become surrounded by a common radiation, which now extends through the whole yolk-substance ; are firmly juxtaposed, and then mutually flattened at the surface of contact ; and finally fuse with each other (fig. 20 fk). The product of their fusion is the first cleavage-nucleus (fk), which undergoes the further alterations leading to cell-division.
This whole interesting process of fertilisation has consumed in the present object of investigation the short time of about ten minutes only.
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Fig. 20. Egg of a Sea-urchin immediately after the close of fertilisation. Egg-nucleus and sperm -nucleus are fused to form the cleavage-nucleus (fk), which occupies the centre of a protoplasmic radiation.
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The phenomena of fertilisation discovered in the Echinoderms were soon observed, either completely or at least partially, in numerous other animals also in Coelenterates and Worms (NusSBAUM, VAN BENEDEN, CARNOY, ZACHARIAS, BOVERI, PLAINER), and in Molluscs and Vertebrates. As regards the last, it has been possible to follow accurately in the case of Petromyzon the penetration of a single spermatozoon into the egg through a special preformed micropyle in the vitelline membrane (CALBERLA, KUPFFER, BENECKE, and BOHM). Likewise in the Amphibia, proof has been brought forward that after fertilisation a sperm-nucleus is formed at the animal pole, and that, surrounded by a pigmented area, derived from the cortex of the yolk, it moves toward another more deeply imbedded nucleus (egg-nucleus), and fuses with it (0. HERTWIG, BAMBEKE, BORN). In Mammals the fertilisation takes place in the beginning of the oviduct. Evidence has also been produced in their case that after the liberation of the polar cells two nuclei are temporarily to be seen in the egg-cells, and that these unite in the centre of the egg to form the cleavage-nucleus (VAN BENEDEN, TAFANI).
This is the proper place in which to mention briefly the so-called micropyle. In many animals (Arthropods, Fishes, etc.) the eggs are enclosed before they are fertilised in a thick firm envelope, which is impenetrable for spermatozoa. Now, in order to make fertilisation possible, there are found in these cases at a definite place on the eggmembrane sometimes one, sometimes several, small openings (micropyles), at which the spermatozoa accumulate in order to glide into the interior of the egg.
The egg of Nematodes has for several years rightly played an important role in the literature of the process of fertilisation. But this is especially true for the egg of the Maw-worm of the Horse (Ascaris megalocephala), which VAN BENEDEN has made the subject of a celebrated monograph. It is an excellent object, in so far as it not only can be had for study everywhere and at all seasons of the year, but also allows one to follow step by step, in the most accurate manner, the penetration and subsequent fate of the spermatozoon. Since, moreover, the process of fertilisation in Ascaris megalocephala presents many peculiarities in its details, an extended presentation of them is both warranted and desirable.
In the case of this Worm, in which the sexes are separate individuals, there is a copulation, and the fertilisation of the egg takes place within the sexual passages of the female. In one region, which is expanded into a kind of uterus, mature spermatic bodies are met with in great numbers. The appearance of these differs greatly from that which the male seminal elements ordinarily present in the animal kingdom : for they are apparently motionless ; are comparable in form to a cone, a conical ball, or a thimble (fig. 21) ; and consist in part of a granular substance (6), in part of a homogeneous lustrous substance (/), and of a small spherical body of nuclear substance (fc), which is imbedded in the granular substance at the base of the cone.
When the small naked eggs enter into the region designated as uterus, fertilisation takes place at once. One spermatic body, which can execute feeble amoeboid motions with its basal end (SCHNEIDER), attaches itself to the surface of the yolk (fig. 22 sk\ Where contact with the egg first takes place, there is formed, exactly as in the Echinoderms, a special cone of attraction. Here the spermatic body, without essential change of form, gradually glides deeper into the yolk, until it is completely / enclosed therein (fig. 23).
While the two sexual products are thus externally fused, the egg itself is not yet ripe, because it still Fig. 21. Spermatic possesses the germinative vesicle (fig. 22 kb), but megaioctphail! ^ now promptly begins to enter upon the maturaafter VAN BENE- tion stage by preparing to form the polar cells. A; Nucleus ; b base The germinative vesicle, which is of small size in of the cone, by the case of the Maw-worm of the Horse, loses its meat to the egg sharp delimitation from the yolk, moves toward takes place; /, that surface of the egg which is opposite to the lustrous substance resembling fat. cone of attraction (figs. 23, 24), and is gradually converted into a nuclear spindle (sp), the origin of which may be traced upon this object with considerable precision. The most important part of the process consists in the formation, out of the chromatic substance, of numerous short, rod-like pieces (figs. 23, 24, ch), which form directly the chromatic elements of the spindle, the chromosomes (WALDEYER). As in the case of the Echinoderms, there then arise at the surface of the yolk two small polar cells (fig. 25 pz) ; as in that case, a vesicular egg-nucleus (fig. 25 ei) arises from the half of the second polar spindle which remains in the peripheral portion of the yolk.
Meanwhile the spermatic body has moved farther and farther from the place of its entrance into the egg (figs. 22, 23, sk), and finally comes to lie in the middle of the yolk (fig. 24 sk), approximately in the position occupied by the germinative vesicle before its migration to the surface. During this period the spermatic body has gradually lost its original form and its sharp delimitation ; out of its nuclear substance, which was described as a small, deeply stainable spherule, there arises a vesicular nucleus (fig. 25 sJc), which acquires the same size and condition as the egg-nucleus.
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Fig. 22. An egg of Ascaris megalocephala just fertilised, after VAX BEXEDEX. sk, Spermatic body, with nucleus, which has entered the egg ; /, fat-like substance of the spermatic body ; kb, germinative vesicle.
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Fig. 23. A stage of a fertilised egg of Ascaris megalocephala, somewhat older than that of fig. 22, after VAX BEXEDEX. sit, Spermatic body, which has penetrated deeper into the cortex of the yolk ; sp, polar spindle, which has arisen from the germinative vesicle ; ch, chromosomes of the spindle.
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Fig. 24. A still older stage of development, following that of fig. 23, of the egg of Ascaris megalocephala, after BOVERI. sp, Polar spindle, which has ascended to the surface of the yolk ; ch, 2 x 4 chromosomes ; sk, spermatic nucleus, which has migrated into the middle of the egg.
Fig. 25. Egg of Ascaris megalocephala in preparation for the process of cleavage, after E. VAX BEXEDEX. pz, Two polar cells which have arisen from the polar spindle (sp) of fig. 24 by a repetition of the process of budding ; ei, egg-nucleus ; sk, spermatic nucleus already preparing to divide ; ch, nuclear loops or chromosomes.
+++++++++++++++++++++++++++++++++++++++++ After the rapid and continuous accomplishment of these processes, the egg of the Worm usually enters on a longer or shorter period of rest. It now presents (compare fig. 25, which represents a stage already further developed) at its surface within the vitelline membrane two polar cells (pz), and in its interior two large vesicular nuclei, the spermatic nucleus (sfc) and the egg-nucleus (ei}, the latter of which has come close up to the former, without, however, fusing with it. A union of the male and female nuclear substances into a common nuclear figure takes place in the case of the Mawworm, when the process of egg-cleavage is beginning.
The processes of fertilisation just described can be designated as typical for the animal kingdom. But they appear to recur in exactly the same manner throughout the vegetable kingdom also, as has been shown by the thorough investigations of STRASBURGER. We are therefore in a better position now than formerly to advance a theory of fertilisation based upon an important array of facts : In fertilisation clearly demonstrable morphological processes take place, Of these the important and essential one is the union of two cell-nuclei which have arisen from different sexual cells, a female eggnucleus and a male spermatic nucleus. These contain the fructifying nuclear substance, which is an organised body and comes into activity as such in fertilisation.
Recently the attempt has been made to expand the fertilisation theory into a theory of transmission. Important reasons may be urged, as appearing to indicate that the fructifying substance is at the same time the bearer of the transmissible peculiarities. The female nuclear substance transmits the peculiarities of the mother, the male nuclear substance the, peculiarities of the father, to the nascent creature. Perhaps there is in this theory a morphological basis for the fact that offspring resemble both progenitors, and in general inherit from both equally numerous peculiarities.
If we accept these two theories, the nucleus, which, despite its constant presence, previously had to be described as a problematic structure of unknown significance, acquires an important role in the life of the cell. It seems to be the cell's especial organ of fertilisation and transmission, inasmuch as there is stored within it a substance (idioplasma of NAGELI) which is less subject to cell metastasis.
In connection with the consideration of the process of fertilisation may be permitted a slight digression to the realm of pathological phenomena.
As follows from numerous observations in both the animal and vegetable kingdoms, in the normal course of fecundation only a single spermatic filament penetrates into an egg, when the encountering sexual cells are entirely healthy. But with an impaired condition of the egg-cell, super fetation by means of two or more seminal filaments (polyspermia) takes place.
Superf etation may be produced artificially, if by way of experiment one injures the egg-cell. Ihis may be accomplished either by exposing it temporarily to a lower or a higher temperature, and thus producing cold-rigor or heat-rigor, or by affecting it with chemical reagents, chloroforming it, or treating it with morphine, strychnine, nicotine, quinine, etc., or by doing violence to it in a mechanical way, such as shaking it. It is interesting to observe how, with all of these means, the degree of superfetation is, to a certain extent, proportional to the degree of the injury ; how, for example, a small number of spermatozoa penetrate into eggs which have been slightly affected with chloral, whereas a greater number penetrate those which have been more strongly narcotised.
In all unfertilised eggs the whole course of development becomes abnormal. But whether, as claimed in FOL'S hypothesis, the origin of double and of multiple organisms is referable respectively to the penetration of two and many spermatozoa, must still be regarded as doubtful. Certainly the question suggested richly deserves to be still more thoroughly tested experimentally.
HISTORY. The facts here given concerning the theory of fecundation are acquisitions of very recent times. To omit the older hypotheses, it was generally assumed up to the year 1875 that the spermatozoa penetrate in great numbers into the substance of the egg, but that they there lose their activity and become dissolved in the yolk.
I succeeded in my study of the eggs of Toxopneustes lividus in finding an object in which all the internal phenomena of fertilisation may be determined with ease and certainty, and in establishing (1) that inconsequence of fertilisation the head of a spermatic filament surrounded by a stellate figure makes its appearance in the cortex of the yolk, and is metamorphosed into a small corpuscle, which I called spermatic nucleus ; (2) that within ten minutes egg-nucleus and spermatic nucleus copulate ; (3) that normally fertilisation is accomplished by only a single spermatic filament, whereas in pathologically altered eggs several spermatozoa may penetrate. I was therefore able at that time to announce the proposition, that fertilisation depends upon the fusion of two sexually differentiated cell-nuclei.
A few months later, VAN BENEDEN announced that in the case of Mammals the segmentation-nucleus arises from the fusion of two nuclei, as had previously been observed by AUERBACH and BUTSCHLI in the case of numerous other objects, and expressed the conjecture that one of them, which has at first a peripheral position, might in part result from the substance of the spermatozoa, which, in great numbers, as he maintained, fuse and become commingled with the cortical portion of the yolk. An advance was soon after this made by FOL, who investigated with the greatest detail the eggs of Echinoderms at the very moment of the penetration of a spermatic filament into the egg, and discovered the formation of a cone of attraction. Since then it has been established by means of numerous researches (those of HELENKA, FOL, HERTWIG, CALBERLA, KUPFFER, NUSSBAUM, VAN BENEDEN, EBERTH, FLEMMING, ZACHARIAS, BOVERI, PLATNER, TAFANI, BOHM, and others) that in other objects also, and in other branches of the animal kingdom, the processes of fertilisation take place in essentially the same manner. At the same time the comprehension of the processes of fertilisation was essentially advanced, especially by the works of VAN BENEDEN on the egg of Ascaris megalocephala, to which have been added the important investigations of BOVERI and others on the same object. STEASBUEGEE has established in a series of excellent researches the identity of the processes of fertilisation in the animal and vegetable kingdoms.
Finally, the phenomena of fertilisation were utilised simultaneously by STEASBUEGEE and myself for the foundation of a theory of heredity, in our endeavor to prove what others (KEBEE, HAECKEL, HASSE) had previously expressed as a conjecture that the male and the female nuclear substances are the bearers of the peculiarities which are transmitted from parent to offspring. KOLLIKEE, Koux, BAMBEKE, WEISMANN, VAN BENEDEN, BOVEEI, and others have since expressed themselves in a similar manner.
Summary
- At maturation the germinative vesicle gradually rises to the animal pole of the egg, and thereby undergoes a regressive metamorphosis (degeneration of the nuclear membrane and the fibrous network, mingling of the nuclear fluid Kernsaft with the protoplasm).
- A nuclear spindle (polar spindle or direction- spindle) is developed out of remnants of the germinative vesicle, principally, indeed, out of the substance of the germinative dot, which breaks up into chromosomes.
- At the place where the spindle encounters the surface of the yolk with one of its ends, there are formed two polar cells or directionbodies (Riehtungskorper) by means of a process of budding, which is repeated.
- At the second budding, half of the nuclear spindle remains in the cortex of the yolk, and is metamorphosed into the egg-nucleus The egg is then ripe.
- In the case of eggs which develop parthenogenetically (Arthropoda), ordinarily only one polar cell is formed.
- At fertilisation only a single spermatozoon penetrates a sound egg (formation of a cone cV attraction, detachment of a vitelline membrane).
- The head of the spermatozoon is converted into the spermatic nucleus, around which the neighbouring protoplasmic particles are radially arranged.
- Egg-nucleus and spermatic nucleus migrate toward each other, and in most instances immediately fuse to form the segmentation nucleus ; in many objects they remain for a considerable time near each other, but not united, and only later are together metamorphosed into the segmentation-spindle.
- In some animals fertilisation of the egg takes place only after completion of its maturation, but in others it is inaugurated at the very beginning of maturation, so that the two phenomena overlap each other.
- Fertilisation theory. Fertilisation depends on the copulation of two cell-nuclei, which are derived from a male cell and a female cell.
- Theory of heredity. The male and female nuclear substances contained in the spermatic nucleus and the egg-nucleus are the bearers of the peculiarities which are transmissible from parents to their offspring.
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Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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