FATHERLESS FROGS

One of the most interesting discoveries of recent date in regard to the processes which go on in that all-important material—protoplasm—which is the physical basis of life and the essential constituent of "cells"—those minute corpuscles of which all living bodies are built—was made in 1910 by a French naturalist, M. Bataillon, and has been examined and confirmed by another French biologist, M. Henneguy. To explain this discovery, a few words as to well-known facts are necessary. It is well known that if we isolate a female frog at the egg-laying season and let her swim in perfectly pure filtered water, and proceed to deposit some of her eggs in that water, the eggs will not germinate; they remain unchanged for a time and then decompose—become, in fact, "rotten." It is a matter of common knowledge that it is necessary for the eggs to be "fertilised" in order that they may start on that series of changes and growth which we call "development," and become tadpoles and eventually young frogs. The "fertilisation" of the frog's eggs is effected in ordinary conditions by the presence in the water of the pond, into which the female sheds them, of microscopic sperm-filaments (often called spermatozoa, or simply "sperms") which are shed into the water at the same time by the male frog.

The egg (the blackish-brown spherical body, as big as a rape-seed, which is imbedded in a thin jelly, and is familiar to those who are drawn by curiosity to look into the waters of wayside ponds in spring) is a single cell or corpuscle of protoplasm distended with dark-coloured and other granules of nutrient substance. A single sperm (though requiring the microscope to render it visible) is also a single cell. It is a minute oval body, with a long serpentine tail of actively undulating protoplasm. Hundreds of thousands of these are shed into the water at the breeding season by the male frog. One is enough to fertilise the egg. The sperm-cells swim in the water, and are chemically attracted by the eggs. As there are so many sperms, one of them is sure to reach each black egg-sphere. It drives its way into the substance of the egg, making a minute hole in its surface; then the protoplasm of the sperm fuses with the protoplasm of the egg, and becomes intimately mixed with it. The egg-cell has a "nucleus," that dense, peculiar, deep-lying, and well-marked "kernel" of its protoplasm which all cells have. It is of essential importance in the life and activity of the cell. The sperm-cell has also a "nucleus," and now (as has been carefully ascertained) the nucleus of the sperm and the nucleus of the egg-cell unite and form one single nucleus. The egg is thereupon said to be "fertilised"—that is to say, "rendered fertile." It at once commences to move. Its surface ripples and contracts and nips in deeply, so that the sphere is marked out into two hemispheres. These are two "cells," or masses of protoplasm, adhering to each other. Each is provided with its own distinct nucleus or cell-kernel, for the first step in the division of the egg-sphere is the division within it of its newly constituted nucleus into two, each half consisting of nearly equal proportions of the mingled substance of the sperm-nucleus and the egg-nucleus. The two first cells or hemispheres again divide, and so the process goes on until the little black egg has the appearance of a mulberry, each granule of the berry being a cell provided with its own nucleus derived from the original nucleus formed by the fusion of the nuclei of the paternal and maternal cells. In the course of a day or two the division has proceeded so far that the resulting "cells" are so small as to be invisible with a hand-glass, and require one to use a high magnifying power in order to distinguish them. And there are hundreds of them; the whole mass of the "egg" within, as well as on the surface, has divided into separate cells. They go on multiplying, take up water, and nourish themselves on the granular nutritive matter present from the first in the egg-cell. The little mass elongates, increases in size, and gradually assumes the form of a young tadpole.

We see, then that the process of fertilisation consists in two things, the latter of which necessitates the former, viz. in the breaking or penetration of the surface of the egg-cell by the active sperm filament and second in the fusion of the substance of the sperm filament with that of the egg in such a way that there is a distinct and intimate fusion of the nucleus of the sperm filament with the nucleus of the egg-cell. The recent discovery of M. Bataillon is this, viz. that you can make the frog's egg develop in a perfectly regular way and become a tadpole and then a young frog without the admission to it of a sperm-filament or of any substance derived from the male frog. All you have to do—and the operation, though it sounds easy and simple, is an exceedingly delicate and difficult one—is to prick with a fine needle the surface of the little black egg-sphere (not merely of the jelly surrounding it) when it is shed by the female frog into perfectly pure water free from sperms or anything of the sort. The slight artificial puncture acts as does the natural puncture by the swimming sperm-filament, and is sufficient! The egg proceeds to develop quite regularly. There is no fusion of the nucleus of the egg-cell with any matter from the outside; no paternal "material" is introduced, but the nucleus of the egg-cell divides just as though there had been! The whole progeny of cells, successively formed, are the pure offspring of the maternal egg-cell and its nucleus. The tadpoles and young frogs so produced are examples of what is called "parthenogenesis"—that is to say, virginal reproduction—reproduction without fertilisation by material derived from a male parent! The needle, which gives off no material, but simply makes a tiny break in the surface of the egg, does all that is necessary!

To those not acquainted with all that has been ascertained as to the reproduction of lower animals such as insects, crustaceans, and worms, this discovery will appear more astonishing than it really is. We know of many lower animals in which the egg-cells produced by the females do regularly and naturally develop without the intervention of a male and without fertilisation. In an earlier volume[7] of this "Easy Chair Series" I wrote of this curious subject, and described the virgin reproduction or parthenogenesis of the hop-louse and other plant lice, of some moths, of some fresh-water shrimps, and of the queen bee (who produces only drones by eggs which are not fertilised). But I had to point out then that no case was known of "parthenogenesis"—that is to say, reproduction by unfertilised eggs—among the whole series of vertebrate animals, the fishes, amphibians, reptiles, birds, and mammals. The chief point of novelty in M. Bataillon's discovery is that we have now an experimental demonstration of parthenogenesis in a vertebrate animal, and in one so highly organised as the frog. And equally interesting, indeed more important from the point of view as to the real meaning and nature of fertilisation, is the mode in which the parthenogenesis of the frog is set going, namely, by a mere prick of the surface film of the ripe egg!

There have, however, been important experiments on the subject of the development of eggs without fertilisation in recent years, prior to these discoveries as to the frog's egg. A favourite subject for such inquiries is the sea urchin (Echinus of different kinds). The female sea urchin, or sea egg, like its close allies the star fishes, lays a great number of very transparent minute eggs (each about the 1/200th of an inch in diameter) in sea-water, and they are there fertilised by the mobile sperm filaments discharged by the males. The eggs are so transparent and so easily kept alive in jars of sea-water that there is no difficulty in watching under the microscope the penetration of the egg by a sperm, and the fusion and other changes in the nuclei. Delages of Paris, and Loeb of California, have made valuable studies on these eggs. Loeb has shown that they may be artificially started on the course of development and cell division without fertilisation—simply by the action of minute quantities of simple chemicals (fatty acids, etc.) introduced into the sea-water by the experimenter. These chemicals appear to act on the delicate pellicle which forms the surface of the egg-cell in much the same way as the prick of a needle acts on a frog's egg. A limited and delicately adjusted disturbance of the cohesion (or of the surface-tension) of the egg-cell seems to be all that is necessary for starting the egg-cell on its career of development. It becomes, in the light of these experiments, not so much a wonder that egg-cells should develop "on their own," but that they do not more frequently do so. It must be remembered that the "germination" and development of unfertilised eggs, even when the whole range of animals and plants is taken into account (for plants also are reproduced by single cells identical in character with the egg-cells and sperm-cells of animals), that is to say, the existence of "parthenogenesis" as a natural, regularly recurring process, is exceptional. We must distinguish cases in which it regularly occurs as part of the life-history of an animal or plant from cases in which it has been successfully brought about by experimental "artificial" methods designed by man. The plant-lice "naturally" reproduce through the summer by unfertilised eggs producing only females, but in the first cold of autumn males are hatched from some of the eggs, and the eggs of this generation are fertilised and bide through the winter, hatching in the following spring. Some few moths and flies also reproduce naturally during summer by unfertilised eggs, and the brine-shrimps and some other fresh-water shrimps produce "fatherless" broods from their eggs, sometimes for years in succession, until "one fine day" some males are hatched, owing to what causes we do not know. The queen bee naturally and regularly lays a certain number of unfertilised eggs, and these produce, not females as do the unfertilised eggs of plant-lice, etc., but male bees—the drones—and it is only from such eggs that the drones of bees are born. These are the chief cases of regular and natural parthenogenesis, but there are others which might be enumerated.

On the other hand, examples of artificially induced development of eggs, not fertilised, are very few. The first known came accidentally to notice. Female silkworm moths reared in confinement sometimes lay eggs when kept apart from the male, and these have been found to hatch, and give rise to caterpillars, which were not reared to maturity. Other moths bred by collectors behaved in the same way, but the grubs were reared to maturity, and three successive generations of "fatherless" moths were obtained. In these cases the hatching of unfertilised eggs is not known to occur in a state of nature, although it probably occurs occasionally. It has also been observed—an important fact when considered with the history of the frog's egg and the needle—that "brushing" the unfertilised eggs of the silkworm and other moths, that is to say, gently polishing the little egg-shells with a soft camel's-hair brush, has the effect of starting development. Taking two lots of unfertilised eggs adhering to slips of paper, as laid by the mother moth, it is found that those gently brushed will hatch, whilst those not brushed will either not hatch at all, or in very small number. The brushing seems to disturb the equilibrium of the protoplasmic egg-cell within the egg-shell just sufficiently to set it going—going on its course of division and development. The only other case of "artificially-induced parthenogenesis" at present recorded is that of the common frog, due to M. Bataillon. There are questions of great interest still to be made out as the result of his discovery. Can the fatherless brood be reared to maturity and again made to yield a fatherless generation? What is the precise structure of the nuclei of the cells which originate from the nucleus of the egg-cell only, and not from a nucleus formed by the fusion of that with a sperm-cell nucleus? These and similar questions are the motive of further careful study now in progress.

The important conclusion is forced upon us by these experiments with a needle, that even in so typical and highly organised a creature as one of the higher or five-fingered, air-breathing vertebrates, the egg-cell does not require any material admixture from the sperm-cell in order that it may successfully germinate and develop, but only a disturbance of equilibrium, which can be administered as well by a needle's point as by a sperm-filament! Yet the whole process of sexual reproduction undoubtedly has, as its origin and explanation, the fusion in the first cell of the new generation from which all the rest will arise, of the material of two distinct individuals. Thus the qualities of the young are not a repetition of the qualities of one parent, nor are they a mere mixture of the qualities of both parents (for contradictory qualities cannot mix). They are a new grouping of qualities comprising some of the one parent and some of the other and hence a great opportunity for variation, for departure from either parent's exact "make-up," is afforded, and for the selection and survival of the new combination. It is, it would seem, only in exceptional cases and for limited periods that uni-sexual or fatherless reproduction can be advantageous to a species of plant or animal. Such cases are those in which abundant food, present for a limited season, renders the most rapid multiplication of individuals an advantage to the species. But after this exceptional abundance has come to an end, the more usual process of reproduction by fertilised eggs (also necessary and advantageous for the preservation of the race by "natural selection in the struggle for existence" of the new varieties so produced) is resumed until again the abundant food is present, as in the annual history of plant lice and the plants on which they feed.