A test fact remains. Sometimes the first polar body extruded undergoes fission while the second is being formed. This can have nothing to do with reducing the number of chromosomes in the ovum. Unquestionably, however, this change is included with the preceding changes in one transaction, effected by one influence. If, then, it is irrelevant to the decrease of chromosomes, so must the preceding changes be irrelevant: the hypothesis lapses. Contrariwise this fact supports the view suggested above. That extrusion of a polar body is a process of cell-fission is congruous with the fact that another fission occurs after extrusion. And that this occurs irregularly shows that the vital activities, seen in cell-growth and cell-multiplication, now succeed in producing further fission of the dwarfed cell and now fail: the energies causing asexual multiplication are exhausted and there arises the state which initiates sexual multiplication.
Maturation of the ovum having been completed, entrance of the spermatozoon, sometimes through the limiting membrane and sometimes through a micropyle or opening in it, takes place. This instantly initiates a series of complicated changes: not many seconds passing before there begins the formation of an aster around one end of the spermatozoon-head. The growth of this aster, apparently by linear rangings of the granules composing the reticulum of the germ-cell, progresses rapidly; while the whole structure hence arising moves inward. Soon there takes place the fusion of this sperm-nucleus with the germ-nucleus to form the cleavage-nucleus, which, after a pause, begins to divide and subdivide in the same manner as cells at large: so presently forming a cluster of cells out of which arise the layers originating the embyro. The details of this process do not concern us. It suffices to indicate thus briefly its general nature.
And now ending thus the account of genesis under its histological aspect, we pass to the account of genesis under its wider and more significant aspects.
CHAPTER VII.
GENESIS
§ 75. Having, in the last chapter but one, concluded what constitutes an individual, and having, in the last chapter, contemplated the histological process which initiates a new individual, we are in a position to deal with the multiplication of individuals. For this, the title Genesis is here chosen as being the most comprehensive title – the least specialized in its meaning. By some biologists Generation has been used to signify one method of multiplication, and Reproduction to signify another method; and each of these words has been thus rendered in some degree unfit to signify multiplication in general.
Here the reader is indirectly introduced to the fact that the production of new organisms is carried on in fundamentally unlike ways. Up to quite recent times it was believed, even by naturalists, that all the various processes of multiplication observable in different kinds of organisms, have one essential character in common: it was supposed that in every species the successive generations are alike. It has now been proved, however, that in many plants and in numerous animals, the successive generations are not alike; that from one generation there proceeds another whose members differ more or less in structure from their parents; that these produce others like themselves, or like their parents, or like neither; but that eventually, the original form re-appears. Instead of there being, as in the cases most familiar to us, a constant recurrence of the same form, there is a cyclical recurrence of the same form. These two distinct processes of multiplication, may be aptly termed homogenesis and heterogenesis.28 Under these heads let us consider them.
There are two kinds of homogenesis, the simplest of them, probably once universal but now exceptional, being that in which there is no other form of multiplication than one resulting from perpetual spontaneous fission. The rise of distinct sexes was doubtless a step in evolution, and before it took place the formation of new individuals could have arisen only by division of the old, either into two or into many. At present this process survives, so far as appears, among Bacteria, certain Algæ, and sundry Protozoa; though it is possible that a rarely-occurring conjugation has in these cases not yet been observed. It is a probable conclusion, however, that in the Bacteria at any rate, the once universal mode of multiplication still survives as an exceptional mode. But now passing over these cases, we have to note that the kind of genesis (once supposed to be the sole kind), in which the successive generations are alike, is sexual genesis, or, as it has been otherwise called —gamogenesis. In every species which multiplies by this kind of homogenesis, each generation consists of males and females; and from the fertilized germs they produce the next generation of similar males and females arises: the only needful qualification of this statement being that in many Protophyta and Protozoa the conjugating cells or protoplasts are not distinguishable in character. This mode of propagation has the further trait, that each fertilized germ usually gives rise to but one individual – the product of development is organized round one axis and not round several axes, Homogenesis in contrast with heterogenesis as exhibited in species which display distinct sexuality, has also the characteristic that each new individual begins as an egg detached from the maternal tissues, instead of being a portion of protoplasm continuous with them, and that its development proceeds independently. This development may be carried on either internally or externally; whence results the division into the oviparous and the viviparous. The oviparous kind is that in which the fertilized germ is extruded from the parent before it has undergone any considerable development. The viviparous kind is that in which development is considerably advanced, or almost completed, before extrusion takes place. This distinction is, however, not a sharply-defined one: there are transitions between the oviparous and the viviparous processes. In ovo-viviparous genesis there is an internal incubation; and though the young are in this case finally extruded from the parent in the shape of eggs, they do not leave the parent's body until after they have assumed something like the parental form. Looking around, we find that homogenesis is universal among the Vertebrata. Every vertebrate animal arises from a fertilized germ, and unites into its single individuality the whole product of this fertilized germ. In the mammals or highest Vertebrata, this homogenesis is in every case viviparous; in birds it is uniformly oviparous; and in reptiles and fishes it is always essentially oviparous, though there are cases of the kind above referred to, in which viviparity is simulated. Passing to the Invertebrata, we find oviparous homogenesis universal among the Arachnida (except the Scorpions, which are ovo-viviparous); universal among the higher Crustacea, but not among the lower; extremely general, though not universal, among Insects; and universal among the higher Mollusca though not among the lower. Along with extreme inferiority among animals, we find homogenesis to be the exception rather than the rule; and in the vegetal kingdom there appear to be no cases, except among the Algæ and a few aberrant parasites like the Rafflesiaceæ, in which the centre or axis which arises from a fertilized germ becomes the immediate producer of fertilized germs.
In propagation characterized by unlikeness of the successive generations, there is asexual genesis with occasionally-recurring sexual genesis; in other words —agamogenesis interrupted more or less frequently by gamogenesis. If we set out with a generation of perfect males and females, then, from their ova arise individuals which are neither males nor females, but which produce the next generation from buds. By this method of multiplication many individuals originate from a single fertilized germ. The product of development is organized round more than one centre or axis. The simplest form of heterogenesis is that seen in most uniaxial plants. If, as we find ourselves obliged to do, we regard each separate shoot or axis of growth as a distinct individual, homogenesis is seen in those which have absolutely terminal flowers; but in all other uniaxial plants, the successive individuals are not represented by the series A, A, A, A, &c., but they are represented by the series A, B, A, B, A, B, &c. For in the majority of plants which were classed as uniaxial (§ 50), and which may be conveniently so distinguished from other plants, the axis which shoots up from the seed, and substantially constitutes the plant, does not itself flower but gives lateral origin to flowering axes. Though in ordinary uniaxial plants the fructifying apparatus appears to be at the end of the primary, vertical axis; yet dissection shows that, morphologically considered, each fructifying axis is an offspring from the primary axis. There arises from the seed a sexless individual, from which spring by gemmation individuals having reproductive organs; and from these there result fertilized germs or seeds that give rise to sexless individuals. That is to say, gamogenesis and agamogenesis alternate: the peculiarity being that the sexual individuals arise from the sexless ones by continuous development. The Salpæ show us an allied form of heterogenesis in the animal kingdom. Individuals developed from fertilized ova, instead of themselves producing fertilized ova, produce, by gemmation, strings of individuals from which fertilized ova again originate. In multiaxial plants, we have a succession of generations represented by the series A, B, B, B, &c., A, B, B, B, &c. Supposing A to be a flowering axis or sexual individual, then, from any fertilized germ it casts off, there grows up a sexless individual, B; from this there bud-out other sexless individuals, B, and so on for generations more or less numerous, until at length, from some of these sexless individuals, there bud-out seed-bearing individuals of the original form A. Branched herbs, shrubs, and trees, exhibit this form of heterogenesis: the successive generations of sexless individuals thus produced being, in most cases, continuously developed, or aggregated into a compound individual, but being in some cases discontinuously developed. Among animals a kind of heterogenesis represented by the same succession of letters, occurs in such compound polypes as the Sertularia, and in those of the Hydrozoa which assume alternately the polypoid form and the form of the Medusa. The chief differences presented by these groups arise from the fact that the successive generations of sexless individuals produced by budding, are in some cases continuously developed, and in others discontinuously developed; and from the fact that, in some cases, the sexual individuals give off their fertilized germs while still growing on the parent-polypedom, but in other cases not until after leaving the parent-polypedom and undergoing further development. Where, as in all the foregoing kinds of agamogenesis, the new individuals bud out, not from any specialized reproductive organs but from unspecialized parts of the parent, the process has been named, by Prof. Owen, metagenesis. In most instances the individuals thus produced grow from the outsides of the parents – the metagenesis is external. But there is also a kind of metagenesis which we may distinguish as internal. Certain entozoa of the genus Distoma exhibit it. From the egg of a Distoma there results a rudely-formed creature known as a sporocyst and from this a redia. Gradually, as this divides and buds, the greater part of the inner substance is transformed into young animals called Cercariæ (which are the larvæ of Distomata); until at length it becomes little more than a living sac full of living offspring. In the Distoma pacifica, the brood of young animals thus arising by internal gemmation are not Cercariæ, but are like their parent: themselves becoming the producers of Cercariæ, after the same manner, at a subsequent period. So that now the succession of forms is represented by the series A, B, A, B, &c., now by the series A, B, B, A, B, B, &c., and now by A, B, B, C, A. Both cases, however, exemplify internal metagenesis in contrast with the several kinds of external metagenesis described above. That agamogenesis which is carried on in a reproductive organ – either an ovarium or the homologue of one – has been called, by Prof. Owen, parthenogenesis. It is the process familiarly exemplified in the Aphides. Here, from the fertilized eggs laid by perfect females there grow up imperfect females, in the ovaria of which are developed ova that though unfertilized, rapidly assume the organization of other imperfect females, and are born viviparously. From this second generation of imperfect females, there by-and-by arises, in the same manner, a third generation of the same kind; and so on for many generations: the series being thus symbolized by the letters A, B, B, B, B, B, &c., A. Respecting this kind of heterogenesis it should be added that, in animals as in plants, the number of generations of sexless individuals produced before the re-appearance of sexual ones, is indefinite; both in the sense that in the same species it may go on to a greater or less extent according to circumstances, and in the sense that among the generations of individuals proceeding from the same fertilized germ, a recurrence of sexual individuals takes place earlier in some of the diverging lines of multiplication than in others. In trees we see that on some branches flower-bearing axes arise while other branches are still producing only leaf-bearing axes; and in the successive generations of Aphides a parallel fact has been observed. Lastly has to be set down that kind of heterogenesis in which, along with gamogenesis, there occurs a form of agamogenesis exactly like it, save in the absence of fecundation. This is called true parthenogenesis – reproduction carried on by virgin mothers which are in all respects like other mothers. Among silk-worm-moths this parthenogenesis is exceptional rather than ordinary. Usually the eggs of these insects are fertilized; but if they are not they are still laid, and some of them produce larvæ. In certain Lepidoptera, however, of the groups Psychidæ and Tineidæ, parthenogenesis appears to be a normal process – indeed, so far as is known, the only process; for of some species the males have never been found.
A general conception of the relations among the different modes of Genesis, thus briefly described, will be best given by the following tabular statement.
This, like all other classifications of such phenomena, presents anomalies. It may be justly objected that the processes here grouped under the head agamogenesis, are the same as those before grouped under the head of discontinuous development (§ 50): thus making development and genesis partially coincident. Doubtless it seems awkward that what are from one point of view considered as structural changes are from another point of view considered as modes of multiplication.29 There is, however, nothing for us but a choice of imperfections. We cannot by any logical dichotomies accurately express relations which, in Nature, graduate into one another insensibly. Neither the above, nor any other scheme, can do more than give an approximate idea of the truth.
§ 76. Genesis under every form is a process of negative or positive disintegration; and is thus essentially opposed to that process of integration which is the primary process in individual evolution. Negative disintegration occurs in those cases where, as among the compound Hydrozoa, there is a continuous development of new individuals by budding from the bodies of older individuals; and where the older individuals are thus prevented from growing to a greater size, or reaching a higher degree of integration. Positive disintegration occurs in those forms of agamogenesis where the production of new individuals is discontinuous, as well as in all cases of gamogenesis. The degrees of disintegration are various. At the one extreme the parent organism is completely broken up, or dissolved into new individuals; and at the other extreme each new individual forms but a small deduction from the parent organism. Protozoa and Protophyta show us that form of disintegration called spontaneous fission: two or more individuals being produced by the splitting-up of the original one. The Volvox and the Hydrodictyon are plants which, having developed broods within themselves, give them exit by bursting; and among animals the one lately referred to which arises from the Distoma egg, entirely loses its individuality in the individualities of the numerous Distoma-larvæ with which it becomes filled. Speaking generally, the degree of disintegration becomes less marked as we approach the higher organic forms. Plants of superior types throw off from themselves, whether by gamogenesis or agamogenesis, parts that are relatively small; and among superior animals there is no case in which the parent individuality is habitually lost in the production of new individuals. To the last, however, there is of necessity a greater or less disintegration. The seeds and pollen-grains of a flowering plant are disintegrated portions of tissue; as are also the ova and spermatozoa of animals. And whether the fertilized germs carry away from their parents small or large quantities of nutriment, these quantities in all cases involve further negative or positive disintegrations of the parents.
Except in spore-producing plants, new individuals which result from agamogenesis usually do not separate from the parent-individuals until they have undergone considerable development, if not complete development. The agamogenetic offspring of those lowest organisms which develop centrally, do not, of course, pass beyond central structure; but the agamogenetic offspring of organisms which develop axially, commonly assume an axial structure before they become independent. The vegetal kingdom shows us this in the advanced organization of detached bulbils, and of buds that root themselves before separating. Of animals, the Hydrozoa, the Trematoda, and the Salpæ, present us with different kinds of agamogenesis, in all of which the new individuals are organized to a considerable extent before being cast off. This rule is not without exceptions, however. The statoblasts of the Plumatella (which play the part of winter eggs), developed in an unspecialized part of the body, furnish a case of metagenesis in which centres of development, instead of axes, are detached; and in the above-described parthenogenesis of moths and bees, such centres are detached from an ovarium.
When produced by gamogenesis, the new individuals become (in a morphological sense) independent of the parents while still in the shape of centres of development, rather than axes of development; and this even where the reverse is apparently the case. The fertilized germs of those inferior plants which are central, or multicentral, in their development, are of course thrown off as centres; and the same is usually the case even in those which are uniaxial or multiaxial. In the higher plants, of the two elements that go to the formation of the fertilized germ, the pollen-cell is absolutely separated from the parent-plant under the shape of a centre, and the egg-cell, though not absolutely separated from the parent, is still no longer subordinate to the organizing forces of the parent. So that when, after the egg-cell has been fertilized by matter from the pollen-tube, the development commences, it proceeds without parental control: the new individual, though remaining physically united with the old individual, becomes structurally and functionally separate: the old individual doing no more than supply materials. Throughout the animal kingdom, the new individuals produced by gamogenesis are obviously separated in the shape of centres of development wherever the reproduction is oviparous: the only conspicuous variation being in the quantity of nutritive matter bequeathed by the parent at the time of separation. And though, where the reproduction is viviparous, the process appears to be different, and in one sense is so, yet, intrinsically, it is the same. For in these cases the new individual really detaches itself from the parent while still only a centre of development; but instead of being finally cast off in this state it is re-attached, and supplied with nutriment until it assumes a more or less complete axial structure.
§ 77. As we have lately seen, the essential act in gamogenesis is the union of two cell-nuclei, produced in the great majority of cases by different parent organisms. Nearly always the containing cells, often called gametes, are unlike: the sperm-cell being the male product, and the germ-cell the female. But among some Protozoa and many of the lower Algæ and Fungi, the uniting cells show no differentiation. Sexuality is only nascent.
There are very many modes and modifications of modes in which these cells are produced; very many modes and modifications of modes by which they are brought into contact; and very many modes and modifications of modes by which the resulting fertilized germs have secured to them the fit conditions for their development. But passing over these divergent and re-divergent kinds of sexual multiplication, which it would take too much space here to specify, the one universal trait is this coalescence of a detached portion of one organism with a more or less detached portion of another.
Such simple Algæ as the Desmidieæ, which are sometimes called unicellular plants, show us a coalescence, not of detached portions of two organisms, but of two entire organisms: the entire contents of the individuals uniting to form the germ-mass. Where, as among the Confervoideæ, we have aggregated cells whose individualities are scarcely at all subordinate to that of the aggregate, the gamogenetic act is often effected by the union "of separate motile protoplasmic masses produced by the division of the contents of any cell of the aggregate. These free-swimming masses of protoplasm, which are quite similar to (but generally smaller than) the agamogenetic 'zoospores' of the same plants, and to the free-swimming individuals of many Protophyta, are apparently the primitive type of gametes (conjugating cells); but it is noteworthy that such a gamete nearly always unites with one derived from another cell or from another individual. The same fact holds with regard to the gametes of the Protophytes themselves, which are formed in the same way from the single cell of the mother individual. In the higher types of Confervoideæ, and in Vaucheria, we find these equivalent, free-swimming, gametes replaced by sexually differentiated sperm- and germ-cells, in some cases arising in different organs set apart for their production, and essentially representing those found in the higher plants. Transitional forms, intermediate between these and the cases where equivalent gametes are formed from any cell of the plant are also known."
Recent investigations concerning the conjugation of Protozoa have shown that there is not, as was at one time thought, a fusion of two individualities, but a fusion of parts of their nuclei. The macro-nucleus having disappeared, and the micro-nucleus having broken up into portions, each individual receives from the other one of these portions, which becomes fused with its own nuclear matter. So that even in these humble forms, where there is no differentiation of sexes, the union is not between elements that have arisen in the same individual but between those which have arisen in different individuals: the parts being in this case alike.
The marvellous phenomena initiated by the meeting of sperm-cell and germ-cell, or rather of their nuclei, naturally suggest the conception of some quite special and peculiar properties possessed by these cells. It seems obvious that this mysterious power which they display of originating a new and complex organism, distinguishes them in the broadest way from portions of organic substance in general. Nevertheless, the more we study the evidence the more are we led towards the conclusion that these cells are not fundamentally different from other cells. The first fact which points to this conclusion is the fact recently dwelt upon (§ 63), that in many plants and inferior animals, a small fragment of tissue which is but little differentiated, is capable of developing into an organism like that from which it was taken. This implies that the component units of tissues have inherent powers of arranging themselves into the forms of the organisms which originated them. And if in these component units, which we distinguished as physiological, such powers exist, – if, under fit conditions, and when not much specialized, they manifest such powers in a way as marked as that in which the contents of sperm-cells and germ-cells manifest them; then, it becomes clear that the properties of sperm-cells and germ-cells are not so peculiar as we are apt to assume. Again, the organs emitting sperm-cells and germ-cells have none of the specialities of structure which might be looked for, did sperm-cells and germ-cells need endowing with properties unlike those of all other organic agents. On the contrary, these reproductive centres proceed from tissues characterized by their low organization. In plants, for example, it is not appendages that have acquired considerable structure which produce the fructifying particles: these arise at the extremities of the axes where the degree of structure is the least. The cells out of which come the egg and the pollen-grains, are formed from undifferentiated tissue in the interior of the ovule and of the stamen. Among many inferior animals devoid of special reproductive organs, such as the Hydra, the ova and spermatozoa originate from the interstitial cells of the ectoderm, which lie among the bases of the functional cells – have not been differentiated for function; and in the Medusæ, according to Weismann, they arise in the homologous layer, save where the medusoid form remains attached, and then they arise in the endoderm and migrate to the ectoderm: lack of specialization being in all cases implied. Then in the higher animals these same generative agents appear to be merely modified epithelium-cells – cells not remarkable for their complexity of structure but rather for their simplicity. If, by way of demurrer to this view, it be asked why other epithelium-cells do not exhibit like properties; there are two replies. The first is that other epithelium-cells are usually so far changed to fit them to their special functions that they are unfitted for assuming the reproductive function. The second is that in some cases, where they are but little specialized, they do exhibit the like properties: not, indeed, by uniting with other cells to produce new germs but by producing new germs without such union. I learn from Dr. Hooker that the Begonia phyllomaniaca habitually develops young plants from the scales of its stem and leaves – nay, that many young plants are developed by a single scale. The epidermal cells composing one of these scales swell, here and there, into large globular cells; form chlorophyll in their interiors; shoot out rudimentary axes; and then, by spontaneous constrictions, cut themselves off; drop to the ground; and grow into Begonias. Moreover, in a succulent English plant, the Malaxis paludosa, a like process occurs: the self-detached cells being, in this case, produced by the surfaces of the leaves.30 Thus, there is no warrant for the assumption that sperm-cells and germ-cells possess powers fundamentally unlike those of other cells. The inference to which the facts point, is, that they differ from the rest mainly in not having undergone functional adaptations. They are cells which have departed but little from the original and most general type: such specializations as some of them exhibit in the shape of locomotive appliances, being interpretable as extrinsic modifications which have reference to nothing beyond certain mechanical requirements. Sundry facts tend likewise to show that there does not exist the profound distinction we are apt to assume between the male and female reproductive elements. In the common polype sperm-cells and germ-cells are developed in the same layer of indifferent tissue; and in Tethya, one of the sponges, Prof. Huxley has observed that they occur mingled together in the general parenchyma. The pollen-grains and embryo-cells of plants arise in adjacent parts of the meristematic tissue of the flower-bud; and from the description of a monstrosity in the Passion-flower, recently given by Mr. Salter to the Linnæan Society, it appears both that ovules may, in their general structure, graduate into anthers, and that they may produce pollen in their interiors. Moreover, among the lower Algæ, which show the beginning of sexual differentiation, the smaller gametes, which we must regard as incipient sperm-cells, are sometimes able to fuse inter se, and give rise to a zygote which will produce a new plant. All which evidence is in perfect harmony with the foregoing conclusion; since, if sperm-cells and germ-cells have natures not essentially unlike those of unspecialized cells in general, their natures cannot be essentially unlike each other.
The next general fact to be noted is that these cells whose union constitutes the essential act of gamogenesis, are cells in which the developmental changes have come to a close – cells which are incapable of further evolution. Though they are not, as many cells are, unfitted for growth and metamorphosis by being highly specialized, yet they have lost the power of growth and metamorphosis. They have severally reached a state of equilibrium. And while the internal balance of forces prevents a continuance of constructive changes, it is readily overthrown by external destructive forces. For it almost uniformly happens that sperm-cells and germ-cells which are not brought in contact disappear. In a plant, the egg-cell, if not fertilized, is absorbed or dissipated, while the ovule aborts; and the unimpregnated ovum eventually decomposes: save, indeed, in those types in which parthenogenesis is a part of the normal cycle.