It is not, however, necessary in all cases that fertilization of the ovum should take place. The plant-lice, or Aphides of our rose trees, may produce generation after generation, and their offspring in turn reproduce in like manner, without any union or fusion of ovum or sperm. The same is true of the little water-fleas, or Daphnids; while in some kinds of rotifers fertilization is said never to occur. It is a curious and interesting fact, which seems now to be established beyond question, that drone bees are developed from unfertilized ova, the fertilized ova producing either queens or workers, according to the nature of the food with which the grubs are supplied. Where, as in the case of aphids and daphnids, fertilization occasionally takes place, it would seem that lowered temperature and diminished food-supply are the determining conditions. Fertilization, therefore, generally takes place in the autumn; the fertilized ovum living on in a quiescent state during the winter, and developing with the warmth of the succeeding spring. In the artificial summer of a greenhouse, reproduction may continue for three or four years without the occurrence of any fertilization.
Fig. 9.—Aurelia: Life-cycle.
a, embryo; b, Hydra tuba; c, Hydra tuba, with medusoid segments; d, medusa separated to lead free existence.]
Mention may here be made of some peculiarly modified modes of reproduction among the metazoa. The aurelia is a well-known and tolerably common jelly-fish. These produce ova, which are duly fertilized by sperms from a different individual. A minute, free-swimming embryo develops from the ovum, which settles down and becomes a little polyp-like organism, the Hydra tuba. As growth proceeds, this divides or segments into a number of separable, but at first connected, parts. As these attain their full development, first one and then another is detached from the free end, floats off, and becomes a medusoid aurelia. Thus the fertilized ovum of aurelia develops, not into one, but into a number of medusæ,[G] passing through the Hydra tuba condition as an intermediate stage.
Many of the hydroid zoophytes, forming colonies of hydra-like organisms, give rise in the warm months to medusoid jelly-fish, capable of producing ova and sperms. Fertilization takes place; and the fertilized ova develop into little hydras, which produce, by budding, new colonies. In these new colonies, again, the parts which are to become ovaries or testes float off, and ripen their products in free-swimming, medusoid organisms. Such a rhythm between development from ova and development by budding is spoken of as an alternation of generations.
The fresh-water sponge (Spongilla) exhibits an analogous rhythm. The ova are fertilized by sperms from a different short-lived individual. They develop into sponges which have no power of producing ova or sperms. But on the approach of winter in Europe, and of the dry season in India, a number of cells collect and group themselves into a so-called gemmule. Round this is formed a sort of crust beset with spicules, which, in some cases, have the form of two toothed discs united by an axial shaft. When these gemmules have thus been formed, the sponge dies; but the gemmules live on in a quiescent state during the winter or the dry season, and with the advent of spring develop into sponges, male or female. These have the power of producing sperms or ova, but no power of producing gemmules. The power of producing ova, and that of producing gemmules, thus alternates in rhythmic fashion.
Fig. 10.—Liver-fluke: Embryonic stages. (After A. P. Thomas.)
A. ovum: em., embryo; op., operculum. B. Limnæus truncatulus (natural size). C. Free embryo: e.s., eye-spot; ex., excretory vessel; g.c., germinal cells; h.p., head-papilla. D. Embryo preparing to become a sporocyst: g.c., germinal cells. E. Sporocyst: g., gastrula; m., morula; re., redia. F. Redia: b.o., birth-opening; ce., cercaria; col., collar; di., digestive sac; ph., pharynx; p.pr., posterior processes; re., daughter redia. G. Cercaria: cys., cystogenous organ; di., digestive sac; o.s., oral sucker; p.s., posterior sucker; ph., pharynx.