by budding. The cut stump of the amputated tentacle of the hydra or the snail buds forth a new organ. But in the hydra, during the summer months, under normal circumstances, a bud may make its appearance and give rise to a new individual, which will become detached from the parent, to lead a separate existence. In other organisms allied to the hydra the buds may remain in attachment, and a colony will result. This, too, is the result of budding in many of the sponges. In some worms, too, budding may occur. In the fresh-water worm (Chatogaster limnai) the animal, as we ordinarily see it, is a train of individuals, one budded off behind the other-the first fully developed, those behind it in various stages of development. The individuals finally separate by transverse division. Another more lowly worm (Microstomum lineare, a Turbellarian) may bud off in similar fashion a chain of ten or fifteen individuals. In these cases budding is not far removed from fission.

Now, in the case of reproduction by budding, as in the hydra, a new individual is produced from some group of cells in the parent organism. From this it is but a step—a step, however, of the utmost importance-to the production of a new individual from a single cell from the tissues of the parental organism. Such a reproductive cell is called an egg-cell, or ovum. In the great majority of cases, to enable the ovum to develop into a new individual, it is necessary that the egg-cell should conjugate or fuse with a minute, active sperm-cell, generally derived from a different parent. This process of fusion of germinal cells is called fertilization (see Fig. 5, p. 13).

In sponges, the cells which become ova or sperms lie scattered in the mid-layer between the ciliated layers which line the cavities and spaces of the organism. Sometimes the individual sponge produces only ova; sometimes only sperms; sometimes both, but at different periods. The cells which become ova increase in size, are passive, and rich in reserve material elaborated by their protoplasm. The cells which become sperms divide again and again,

and thus produce minute active bodies, adance with restless motion. These opposite tendencies are repeated and emphasized throughout the animal kingdom-ova relatively large, passive, and accumulative of reserve material; sperms minute, active, and the result of repeated fission. The active sperm, when it unites with the ovum, imports. into it a tendency to fission, or cleavage; but the resulting cells do not part and scatter-they remain associated together, and in mutual union give rise to a new sponge. In the hydra, generally near the foot or base of attachment, a rounded swelling often makes its appearance in

A, hydra half retracted, with a bud and an ovum attached to the shrunken ovary; B, a small bydra firmly retracted; C, a hydra fully extended. b., bud; f., foot; h.s., hypostome; OC., ovun; ovy., ovary; t., tentacles; ts., testis.

autumn. Within this swelling one central cell increases enormously at the expense of the others. It becomes an ovum. Eventually it bursts through the swelling, but remains attached for a time. Rarely in the same hydra, more frequently in another, one or two swellings may be seen higher up, beneath the circle of tentacles. Within these, instead of the single ovum may be seen a swarm of sperms, minute and highly active. When these are discharged, one may fuse with and fertilize an ovum, occasionally in the same, but more frequently in another individual, with the result that it develops into a new hydra. Here there are definite organs-an ovary and a

testis-producing the ova or the sperms. But they are indefinite and not permanent in position.

In higher forms of life the organs which are set apart for the production of ova or sperms become definite in position and definite in structure. Occasionally, as in the snail, the same organ produces both sperms and ova, but then generally in separate parts of its structure. The two products also ripen at different times. Not infrequently, as in the earthworm, each individual has both testes and ovaries, and thus produces both ova and sperms, but from different organs. The ova of one animal are, however, fertilized by sperms from another. But in the higher invertebrates and vertebrates there is a sex-differentiation among the individuals, the adult males being possessed of testes only and producing sperms, the adult females possessed of ovaries only and producing ova. There are also, in many cases, accessory structures for ensuring that the ova shall be fertilized by sperms, while sexual appetences are developed to further the same end. But however the matter may thus be complicated, the essential feature is the same-the union of a sluggish, passive cell, more or less laden with nutritive matter, with a minute active cell with an hereditary tendency to fission.*

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

* Professor Geddes and Mr. J. Arthur Thomson, in their interesting work on "The Evolution of Sex," regard the ovum in especial, and the female in general, as preponderatingly anabolic (see note, p. 32); while the sperm in especial, and the male in general, are on their view preponderatingly katabolic. Regarding, as I do, the food-yolk as a katabolic product, I cannot altogether follow them. The differentiation seems to me to have taken place along diver gent lines of katabolism. In the ovum, katabolism has given rise to storage products; in the sperm, to motor activities associated with a tendency to fission. The contrast is not between anabolic and katabolic tendencies, but between storage katabolism and motor katabolism. Nor do I think that "the essentially katabolic male-cell brings to the ovum a supply of characteristic waste products, or katastates, which stimulate the latter to division" (l.c., p. 162). I believe that it brings an inherited tendency to fission, and thus reintroduces into the fertilized ovum the tendency which, as ovum, it had renounced in favour of storage katabolism.

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.

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

a, embryo; b, Hydra tuba; c, Hydra tuba, with medusoid segments; d, medusa separated to lead free existence.

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 medusa, 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 freeswimming, 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 dises 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.

But one more example of these modified forms of reproduction can here be cited (from the author's text-book on "Animal Biology"). The liver-fluke is a parasitic organism, found in the liver of sheep. Here it reaches sexual maturity, each individual producing many thousands of eggs, which pass with the bile into the alimentary canal of the host, and are distributed over the fields with the *On the other hand, three ova of the crustacean Apus are said to coalesce to form the single ovum from which one embryo develops.