Parthenogenesis

“ Parthenogenesis is no longer wonderful, and if we did not know that great good followed from the union of the sexual elements derived from two distinct individuals, the wonder would be that parthenogenesis did not occur much oftener than it does. On any ordinary theory of reproduction the formation of graft-hybrids, and the action of the male element on the tissues of the mother-plant, as well as on the future progeny of female animals, are great anomalies; but they are intelligible on our hypothesis. The reproductive organs do not actually create the sexual elements ”

“ The phenomenon of parthenogenesis and the recent work on artificial parthenogenesis in animal types show that the energy in the egg may be liberated and the wheels of development set in motion by the proper chemical and physical environment without the intervention of the sperm. ”

“ We know further that a similar process, but much more highly developed, is possible in what is called parthenogenesis, or virgin-birth, which can only be explained as a survival of the early form. For long life continued without the assistance of the male-cell, which, when it did arise, was dependent on the ova, or female-cell, and was driven by hunger to unite with it in fatigue to continue life. ”

“ Parthenogenesis effects a very considerable increase in the fertility of a species, and in this increase the reason for its introduction among natural phenomena obviously lies. By the occurrence of parthenogenesis, the number of ova produced by a particular colony of animals may be doubled, because each individual is a female, and as the multiplication increases in geometrical ratio a few parthenogenetic generations result in a number of descendants enormously in excess of those produced by bi-sexual reproduction. ”

“ Experiments on artificial parthenogenesis have been much written about in the daily press, and many absurd things concerning this topic have been printed in the newspapers. Ordinarily the ovum requires to be fertilized in order to develop, but it has long been known that certain ova, of bees, of plant lice, of some Crustacea and of other animals will develop without being fertilized. ”

“ Such nuclei cannot control the whole cell-body and force it to divide, but each one of them, having grown to a certain size at the expense of the cell-body, makes its influence felt over a certain area. Strasburger is quite right in considering this process as a ‘partial parthenogenesis.’ Such partial parthenogenesis presumably occurs in all egg-nuclei, but the latter cannot attain to complete parthenogenesis when, as in Fol’s supernumerary sperm-nuclei, their powers of assimilation are insufficient to enable them to reach the requisite size. ”

“ O. Hertwig had defined fertiliza­tion as consisting in the fusion of two nuclei, the egg nucleus and the sperm nucleus. No such fusion of two nuclei takes place in artificial parthenogenesis since no spermato­zoön enters the egg, and it became necessary, therefore, to abandon Hertwig’s defini­tion as wrong. The objec­tion raised that the phenomena are limited to a few species soon became untenable since it has been possible to produce artificial parthenogenesis in the egg of plants (Fucus, according to Overton) as well as of animals, from echinoderms up to the frog ”

“ No case is yet known of a species exhibiting the three forms of parthenogenesis. From this review we may conclude that parthenogenesis does not favour the formation of one sex more than another; but it is clear that it decidedly favours the production of a brood that is {499}entirely of one sex, but which sex that is differs according to other circumstances. ”

“ Parthenogenesis takes place when the whole of the ancestral germ-plasms, inherited from the parents, are retained in the nucleus of the egg-cell. Development by fertilization makes it necessary that half the number of these ancestral germ-plasms must be first expelled from the egg, the original quantity being again restored by the addition of the sperm-nucleus to the remaining half. ”

“ But Loeb4 succeeded in obtaining a full development by treating the eggs of echinoderms with chloride of magnesium; thus artificial parthenogenesis had been discovered. Later researches have shown that artificial parthenogenesis may occur in all classes of the animal kingdom and may be provoked by all sorts of chemical or physical means. We do not know at present in what the proper stimulus consists that must be supposed here to take the place of fertilisation ”

“ In these, indeed, parthenogenesis does occasionally occur, that is to say, sexually differentiated female germ-cells are, by means of some slight variations in the maturation of the egg, rendered capable of developing without previous amphimixis, but this happens only in quite special cases as an adaptation to quite special circumstances, and can only be regarded as a temporary cessation of the association between reproduction and amphimixis. ”

“ The egg-cells and sperm-cells cannot be very quiescent since they do so much when they unite. Moreover, we have simply to recall the facts of natural parthenogenesis on the one hand or of artificial parthenogenesis on the other, to see that the quiescence of the egg is a secondary restriction adapted to secure amphimixis. Moreover, the familiar external and internal changes which occur in the bodies of organisms when they are approaching sexual maturity suggest the very opposite of general or molecular equilibrium. ”

“ In some forms of {142}parthenogenesis the young are produced alive instead of in the form of eggs. A very rare kind of parthenogenesis, called paedogenesis, has been found to exist in two or three species of Diptera, young being produced by the immature Insect, either larva or pupa. ”

“ These multiply very rapidly through the summer by parthenogenesis, generation after generation, and they differ from the females which are fertilized by a male in many features, all of which are of such a character as to render the parthenogenetic females unusually fertile. ”

“ If at every egg-laying the arrival of a male had to be waited for, an even larger number of females would fall victims to the unfavourable weather and other dangers, while at the same time the number of emerging females could be only half as large as it is. It is obvious that in this case parthenogenesis is of very great advantage. ”

“ Parthenogenesis, or development of the female gamete without fertilization, is known to occur in many groups of the animal kingdom. Attempts have been made to connect this phenomenon with peculiarities in the gametogeny. For instance, it has been said that parthenogenetic ova Parthenogenesis. form only one polar body. But, as we have seen, this is sometimes the case in eggs which are fertilized, and parthenogenetic ova are known which form two polar bodies, e.g. ”

“ All the facts with which we are acquainted confirm this conclusion, for whole groups of purely parthenogenetic species or genera are never met with, as would certainly be the case if parthenogenesis had been the only method of reproduction through a successional series of species. We always find it in isolated instances, and under conditions which compel the conclusion that it has become predominant in the species in question, and has not been transmitted from any preceding species. ”

“ For here we may be approaching to the most complete exemplification of Parthenogenesis, exhibited in the case of certain groups of insects, where the species is constituted, solely and at all periods, by females producing perfect eggs, and undistinguished, as it seems,by any visible peculiarity either of redundancy or defect in the reproductive organs. ”

“ It is quite probable that parthenogenesis is more common than has been supposed, and it appears that there is some evidence to show that ova, which in normal conditions are incapable of developing without fertilization, may yet develop if subjected to an altered environment. ”

“ They couple and produce females parthenogenetically endowed. This has been for long time a mystery,—it is still a mystery, for side by side with normal parthenogenesis there is irregular parthenogenesis, there are cases where non-fecundated eggs behave exactly as fecundated eggs, without anyone's knowing why.
The virgin-begotten cycle of plant-lice is famous, that of wheel-animals not less entertaining. The males, smaller than the females, live but two or three days, couple and die. ”

“ Parthenogenesis, in the production of males, has also been found by Siebold to be true of other bees and wasps, and of some of the lower moths, in the production of both males and females. While the great Bonnet first discovered what may be noticed on any summer day, all about us, even on the house-plants at our very windows, that parthenogenesis is best illustrated by the aphides or plant lice. In the fall males and females appear, which mate, when the female lays eggs, which in the spring produce only females ”

“ The phenomena of this demonstrate that the female element is the highest in evolution. Spitzka [67] has shown that the ovum possesses an inherent activity independently of fructification. How far this may extend in the direction of more mature development is shown by what is known about parthenogenesis. This is the development of living beings without a father. Bees, some butterflies, ants and wasps notoriously multiply their kind without sexual congress. As a rule the parthenogenetic offspring are themselves incapable of further procreating their kind. ”

“ Among arthropodes, that is to say among insects in general, the sexes are always separate, save in certain tardigrade arachnids, but these are the ones which offer the finest cases of parthenogenesis, generation without aid of the male. The term need not be taken literally. For as there is no indefinite scissiparity without coupling, there is no unlimited parthenogenesis without fecundation: the female is fecundated for several generations which transmit this power, but there comes a day when the female who has not encountered a male gives birth to males and females. ”

“ Moreover, the now well-ascertained cases of Parthenogenesis prove that the distinction between sexual and asexual generation is not nearly so great as was formerly thought; for ova occasionally, and even in some cases frequently, become developed into perfect beings, without the concourse of the male. With most of the lower animals and even with mammals, the ova show a trace of parthenogenetic power, for without being fertilised they pass through the first stages of segmentation. ”

“ In every case in which parthenogenesis has been first introduced in alternation with sexual reproduction, the resting eggs are produced by the latter generations, while the parthogenetic generations produce eggs with thin shells, in which the embryo developes and hatches very rapidly. ”

“ It is said that in a few species of animals males do not occur, and that parthenogenesis is the sole means of reproduction (a species of Ostracoda among Crustacea; species of Tenthredinidae, Cynipidae and Coccidae among Insecta) ; this is the thelytoky of K.T.E. von Siebold. The number of species in which males are unknown is constantly decreasing, and it is quite possible that the phenomenon does not exist. Parthenogenesis, however, is undoubtedly of frequent occurrence, and is of four kinds, namely, (1) that in which males alone are produced, e.g. ”

“ Whatever the details of development, one cardinal fact is clear. Except for the relatively rare instances of parthenogenesis a new individual, whether plant or animal, arises as the joint product of two sexual cells derived from individuals of different sexes. Such sexual cells, whether ovules or ova, spermatozoa or pollen grains, are known by the general term of gametes, or marrying cells, and the individual formed by the fusion or yoking together of two gametes is spoken of as a zygote. ”

“ During the summer there are strictly no males nor females, but all are sexually alike, and are able to produce ova which develop without fecundation, this exceptional method of reproduction being termed parthenogenesis. ”

“ A sexual female produces one or two winter-eggs, and if these are fertilised they are enclosed in an ephippium and cast off; if, however, the eggs when ripe are not fertilised, they atrophy, and the female produces parthenogenetic eggs, being thenceforward incapable of forming sexual “winter” eggs. An accidental absence of males may thus lead to the occurrence of parthenogenesis in the whole of the second generation. The regular production of sexual individuals in the second generation from the winter-egg appears to depend on a variety of circumstances not yet understood. ”

“ Amongst insects it occurs in many forms in many different groups, sometimes occasional, sometimes as a regular occurrence. Apart from Aphides the classical instance is that of the bee, where eggs that are not fertilized develop parthenogenetically and produce only drones. What is known as pathological parthenogenesis has been observed occasionally in higher animals, e.g. the frog, the fowl and certain mammals, whilst in the case of human beings, ovarian cysts in which hair and other structures are produced have been attributed to the incomplete development of parthenogenetic ova. ”

“ La fécondation est la condition de la continuité de la vie. Par elle le générateur échappe à la mort’ (l. c., p. 405) . Victor Hensen thinks it possible that the germ and its products are prevented from dying by means of normal fertilization: he says that the law which states that every egg must be fertilized, was formulated before the discovery of parthenogenesis and cannot now be maintained, but that we are nevertheless compelled to assume that even the most completely parthenogenetic species requires fertilization after many generations (l. c., p. 236) . ”