New stocks may also have their origin from such buds, or from single persons of the stock. The fresh-water polyp (Hydra) gives rise by budding to a small stock of at most three or four persons; but the young animals budded off only remain attached to the parent hydra until they have attained their full development; then they detach themselves and settle down independently, and begin to bud off in turn a similar and transitory stock. Among plants there are many which, like Dentaria bulbifera and Marchantia polymorpha, multiply by so-called 'brood-buds,' that is, buds which fall from the stock and grow into new plants. The whole horticultural propagation of plants by cuttings also depends on the process of budding, for what is cut off from the parent plant and stuck into the earth is a single shoot, that is, a 'person' which possesses the power of sending down roots into the earth, and by continual budding giving rise to new shoots or persons which together make up a new plant-stock.
I must not, however, spend much time over this so-called 'asexual' reproduction by budding and division, because it does not suggest any way by which we may penetrate more deeply into the processes of inheritance, and we may be content if we can bring them into harmony with other theoretical views which we deduce from other phenomena. These forms of reproduction were long regarded as the oldest and the simplest, and it is only since the time of Francis Balfour that the conviction has gradually gained ground that this cannot be so, but that they are rather secondary methods of multiplication in the Metazoa and Metaphyta, which therefore rest on a very complex basis. We have seen that the germ-cells made their appearance along with the multicellular body, and the step from Pandorina to Volvox is as small a step as can be well imagined. It is thus proved that the oldest mode of multiplication among multicellular organisms was that through germ-cells, at least along this line of evolution. Volvox does not reproduce by dividing, or by the development of buds from any part of the spherical colony of cells. What is known as budding among single-celled organisms is only an unequal cell-division, and has nothing but its external appearance in common with the budding of higher plants and animals. The latter, therefore, is something new, of later and independent origin; the primitive mode is reproduction by unicellular germs.
LECTURE XIV
REPRODUCTION BY GERM-CELLS.
Historical—Differentiation of germ-cells into male and female—Pandorina—Volvox—Sperm-cells and ova in Algæ—Zoosperm form of the male germ-cells—Zoosperms of the Barnacles—Adaptation of the sperm-cells to the conditions of fertilization—Daphnids—Spermatozoa in different animal groups—Their minute structure—Form and structure of the egg-cell—Adaptation of the ovum to given conditions—Dimorphic ova in the same species—Nutritive cells associated with egg-cells—Complex structure of the bird's egg.
If we now turn to the reproduction of the Metazoa and Metaphyta by means of germ-cells we find that a great number of lowly plants produce germ-cells which require nothing more for the development of a new plant beyond certain favourable external conditions, above all, moisture and warmth. Such, for instance, are the 'spores' of the ferns, which are formed on the under surface of the fronds in little clusters of a brown or yellow colour, easily visible to the naked eye. These spores are individually very small, so that thousands go to form one spore-cluster or sporangium, and millions of spores are given off annually by a single fern. Each spore is a germ-cell enclosed in a protective capsule, and may, if carried by the wind to a spot favourable to germination, become a young plant, the so-called prothallium, from which the fern-plant proper subsequently develops.
This reproduction by spores has been regarded as a form of 'asexual reproduction' so-called, and has been classed along with budding and fission under this head. But it has nothing in common with these forms of multiplication except the negative character that the act of fertilization, which we shall inquire into later on, does not in this case occur. This mode of classification has no longer any more justification than the division of the animal kingdom into backboned and backboneless animals, in which the negative character of the absence of vertebræ has led to the slumping of quite heterogeneous forms in one group. I do not mean to dispute that both these classifications were fully justified in their own time; indeed they expressed a step of progress. Nowadays, however, the division 'Invertebrata' or 'backboneless animals' as a scientific conception has been abandoned, and the same should be done with the category 'asexual reproduction,' since it groups together quite different things, such as multiplication by single-celled and many-celled 'germs,' and is moreover based on a quite erroneous idea of what 'fertilization' really is. Both terms may very well be retained as a mere matter of convenience, but it is much to be desired that the two apt designations proposed by Haeckel—Monogony for asexual, and Amphigony for sexual reproduction—should come into general use.
Meanwhile it is enough to say that reproduction by 'spores' occurs normally in Algæ, fungi, mosses, and fern-like plants, and that there are also animals in which the germ-cells possess the power of giving rise of themselves to a new individual. But the cases which I am chiefly thinking of are those of so-called virgin birth or parthenogenesis, which are not to be compared with multiplication by spores in regard to their mode of origin; there is a peculiarity in the origin of this mode of multiplication which I can only make clear after we have studied the normal forms of what is called 'sexual reproduction.'