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.
[7] "Science from an Easy Chair," Methuen & Co., 1910.
CHAPTER XIV
PRIMITIVE BELIEFS ABOUT FATHERLESS PROGENY
In the preceding chapter I related the curious and exceptional cases of "fatherless reproduction" by means of true egg-cells, those cells of special nature produced in the organs called "ovaries," present in all but the simplest animals and plants. These egg-cells are usually, with elaborate sureness and precise mechanism after liberation from the ovary, fertilised by (that is to say, fused with) the complemental reproductive cells—the sperm-filaments—produced by other individuals, the males.