In order to appreciate this matter it is necessary to know the chief facts about the ordinary process of reproduction in animals and plants. All animals and plants are built up of minute particles of living matter called “cells” (see [p. 170]). Really, these are not cells, or hollow boxes, or cases. We use the word “cell” for the contents of a cell. Each is a droplet of protoplasm or living matter lying in a small or large envelope or case of dead matter which it has produced around itself ([Fig. 61]). Observers using their microscopes saw at first only the case, and called it a “cell,” and the word “cell” is now used almost universally for the soft stuff within the cell (see [p. 173]). Each soft cell of “plasm” or “protoplasm” has a very special structure. The existence in it of a central kernel, or “nucleus” of peculiarly active substance, is the most obvious feature. These “cells” are so small (for instance, those which build up the human body) that from one to two thousand could be placed side by side on a line an inch long. They are the “units” which make up the body of an animal or plant, just as bricks and planks and rods make up a building constructed by human contrivance. Two most important things about them are—first, that each is always the seat of chemical activity, absorbing liquid material, changing it and either fixing it or throwing it out in a new chemical condition; and, second, that as a result each cell grows, and after a very little growth divides into two. This “dividing into two” is immensely important, for in this way the number of cells forming a very young or small animal or plant is increased from a few thousands to many millions whilst the organism grows. And not only that, but we find on tracing the young animal or plant back to its beginning as an individual that it actually started as a single cell. The germ of every living thing, then, is a single nucleated particle of protoplasm—a cell which we call the “egg-cell,” because “eggs” are merely shells and packing to hold and protect this all-important egg-cell.
Fig. 61.—A single egg-tube or ovarian tube (usually there are many) of an insect. The youngest and smallest eggs are at the narrow end. o o are larger egg-cells with a striated shell or envelope; g, nucleus of the egg-cell. The unshaded egg is one grown to full size, and in the parthenogenetic aphis would develop where it is without fertilisation into a young aphis.
Every individual flower, tree, insect, snail, fish, and man started as a single egg-cell, which became detached from the mother’s body. Take the case of a common marine animal, the star-fish. At the breeding season, early in the year, the female star-fish discharges thousands of these egg-cells into the sea-water. Each floats separately in a delicate case of its own. Before any one of those floating egg-cells can commence to divide so as to build up a new mass of cells—a new young star-fish—it must undergo the process of “fertilisation.” That is to say, its substance must fuse with that of a “sperm-cell.” These “sperm-cells” are discharged into the sea-water in countless thousands by the male star-fishes. They are excessively minute, actively wriggling threads, swollen out at one end to form a little knob, the “nucleus” of the sperm-cell (see [p. 134] for figures of the spermatozoa, and eggs of the oyster). The water is rendered cloudy by the abundance of these microscopic filaments, which are called “spermatozoa.” One sperm-cell, or spermatozoon comes into contact, in the sea-water, with each of the discharged floating egg-cells. It burrows into it and fuses or melts and mixes with the substance of the egg-cell. The whole process is easy to watch with a microscope, and I am writing of what I, in common with many others, have actually seen.
The egg-cell after this process consists really of the substance of two equal cells—the egg-cell and the sperm-cell—completely fused so as to form a single cell, having a single “nucleus,” which has resulted from the fusion of the nucleus of the egg-cell with that of the sperm-cell. Now, and not before, the egg-cell can divide, take up nourishment, and continue to divide and grow, so as to form a constantly increasing mass of young cells, a young animal which gradually assumes the form of a star-fish. All animals, and plants, too, reproduce themselves in this way. When the animal or plant is not aquatic in its habits the sperm-cell and the egg-cell cannot be discharged and take their chance of coming into contact with one another outside the parent’s body; the sperm-cells are, in such cases, received into a chamber of the egg-producing parent’s body, and there the fusion of the egg-cells with them, one sperm-cell to one egg-cell, takes place. Parthenogenesis then consists in the omission of the fusion of a sperm-cell with the egg-cell. The egg-cell develops, divides again and again, and produces the young animal without the addition to it of a sperm-cell—without, in fact, being “fertilised,” as it is called. That is what happens in the summer broods of the little plant-lice or aphides ([Fig. 57]). When, however, the cold weather comes the virgin mothers suddenly produce two kinds of young—males as well as females—and then the solitary winter egg, which the late autumn females lay to last through winter until spring, is fertilised by a sperm-cell derived from the late produced autumn male ([Fig. 56]) in the ordinary way.
Another parthenogenetic animal is the rare little fresh-water shrimp called Apus, which goes on multiplying in this manner in wayside ponds for years, thousands of female individuals being produced in successive seasons, laying their eggs and carrying on the race for an indefinite time until at last—one fine day—we do not know why then and not before, that rare creature a male Apus is hatched. Why these and one or two other such small shrimps and insects are able to set aside the almost universal law as to the necessity for fertilisation of the egg-cell by a sperm-cell, naturalists have not yet found out. It is quite certain that these exceptional creatures have been derived from ancestors which had their eggs fertilised in the regular way, and that this elimination of the male is a special device, an innovation.
There are incomplete attempts at it in other insects. Thus it has been discovered that the queen bee produces only females from the eggs which are fertilised before she lays them. When the stock of sperm-cells which she received from a drone in her nuptial flight is exhausted, or if we carefully remove by a painless operation the internal sac in which they are stored, the eggs are no longer fertilised, but they are not rendered sterile or abortive. They develop into drones! And drones or male bees are produced in no other way, and only drones are so produced, never worker-females (so-called neuters) nor queens.
Another curious fact is that in rearing moths in captivity some naturalists have quite unexpectedly found that when they have hatched out female moths from the chrysalids and kept them from the moment of hatching quite apart from the male moths (which are of another size and colour, and easily distinguished), these females will sometimes lay eggs—unfertilised eggs—which give birth to caterpillars, which feed and complete all their changes. The second generation of moths so produced are male and female, but the females, being kept apart again, produce a parthenogenetic brood, and the process has been repeated to a third generation. These instances are very rare. The remarkable thing about them is that, apparently, the parthenogenesis is only due to the experimental interference of an entomologist, and that unless some such accident had befallen the moths, the eggs would have been fertilised in the usual way, since there was no deficiency of male moths. These facts have led to many interesting speculations, and are particularly curious in regard to the inquiry as to what determines the sex of offspring, about which sensational announcements are sometimes made in the foreign correspondence columns of our newspapers. Here we find the parthenogenetic eggs of the moths producing both males and females, those of the aphides and the pond-shrimp producing predominantly females, and those of the queen bee producing exclusively males (drones). Biologists have not yet arrived at a solution of the problem raised by these divergent results.