Corroborative evidence appears in the contrast which persists in all the higher forms between the relatively large female-cell or germ and the microscopical male-cell or sperm, as also in the absorption of the male cellule by the female cellule. In the sexual cells there is no character in which differentiation goes so far as that of size.[24] The female cell is always much larger than the male; where the former is swollen with the reserve food, the spermatozoa may be less than a millionth of its volume. In the human species an ovum is about 3000 times as large as spermatozoa.[25] The male cellule, differentiated to enable it to reach the female, impregnates and becomes fused within her cellule, which, unlike hers, preserves its individuality and continues as the main source of life. It is true that exceptions occur, sex-parasitism appearing in both sex forms, and in some cases it is the female who degenerates and becomes wholly passive and dependent, but this is usually under conditions which afford in themselves an explanation. Thus, in the troublesome thread-worm (_Heterodera schachtii_), which infests the turnip plant, the sexes are at first alike, then both become parasitic, but the adult male recovers himself, is agile and like other thread-worms, while the female remains a parasitic victim without power of function--a mere passive, distended bag of eggs. Another extreme but well-known example is that of the cochineal insect, where the female, laden with reserve products in the form of the well-known pigment, spends much of its life like a mere quiescent gall on the cactus plant; the male, on the other hand, is active, though short-lived. Among other insects--such, for example, as certain ticks--a very complete form of female parasitism prevails; and while the male remains a complex, highly active, winged creature, the female, fastening itself into the flesh of some living animal and sucking its blood, has lost wings and all activity and power of locomotion, having become a mere distended bladder, which, when filled with eggs, bursts and ends a parasitic existence that has hardly been life.[26] In many crustaceans, again, the females are parasitic, but this also is explained by their habit of seeking shelter for egg-laying purposes.[27]