[Illustration: 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.