The exceptional and artificially produced cases of parthenogenesis which we have discussed above are probably to be interpreted thus: through slight differences in the constitution of the ovum, or through certain mechanical or chemical stimuli, the metabolic processes in the ovum are so altered that the centrosome of the ovum, instead of breaking up, is stimulated to growth, and thus produces the active dividing apparatus which is otherwise only brought into it by the sperm. This is a more exact definition of the interpretation I gave earlier (1891) of the 'chance' parthenogenesis of the silk-moth, which was then the only case known, when I said 'the nucleoplasm of some ova must possess the power of growth in a greater degree than the majority.'
But we are not yet in a position to go further, or to define more exactly the nature of the processes of metabolism which are involved.
LECTURE XVI
FERTILIZATION IN PLANTS AND UNICELLULAR
ORGANISMS, AND ITS IMMEDIATE
SIGNIFICANCE
Fertilization in a lichen, Basidiobolus—In Phanerogams—Here too there is reduction of the number of chromosomes by a half—'Polar cells' in lower and higher plants—Conjugation among unicellular organisms—Noctiluca—The maternal and paternal chromosomes remain apart—Actinophrys—Infusoria—Sexual differentiation of the two conjugates in Vorticella—Importance of the process of Amphimixis—Not a 'life-awakening' process—May occur independently of multiplication—The Rejuvenescence hypothesis—Pure parthenogenesis—The cycle idea—Does Amphimixis prevent natural death?—Maupas' experiments with Infusorians—Bütschli's view—Potential immortality of unicellular organisms—The immortality of unicellular organisms and of the germ-cells depends on the fact that there is no time-limit to the multiplication of the smallest living particles—Parthenogenesis is not self-fertilization—Petrunkewitsch's observations on the ova of bees—Is the chromatin really the 'hereditary substance'?—Nägeli's conclusion from the difference in size between ovum and spermatozoon—Artificial division of Infusorians—Boveri's experiments with the fertilization of pieces of ova not containing a nucleus—Fertilization gives an impulse to development even to non-nucleated pieces of ova—Merogony—The female and male nuclear substances are essentially alike—Summary.
I now turn to the consideration of the process of fertilization in plants and unicellular organisms.
With regard to plants, it can now be definitely asserted that in them, too, fertilization is essentially a conjugation of nuclei; it depends on the union of the nuclei of the two 'sex-cells.' These sex-cells are usually very small among lower plants, indeed up to the phanerogams; this is especially true of the zoosperm-like male germ-cells, but it usually holds also true of the ovum, which is but seldom burdened with an abundant supply of yolk. In spite of the many difficulties which this smallness of size puts in the way of observation, the untiring exertions of a host of excellent investigators have succeeded in following the process of fertilization in all the larger groups of plants—in algæ, fungi, mosses, ferns, and horse-tails among cryptogams, and in phanerogams.
I shall first give an example from among the lower plants (Fig. 81). In one of the lichens, Basidiobolus ranarum, each of two adjacent cells in the fungus-thread gives off a bill-like process, and the two processes become closely apposed (Fig. 81, a). The nucleus of each cell moves into the bill-shaped process, is there transformed into a nuclear spindle (B, ksp) and divides, so that one daughter-nucleus comes to lie in the apex point of the bill, the other at the base. The cell-body also divides, though very unequally, and the final outcome of the process is two cells in each, of which one is small and occupies the apex of the bill, while the other is large and fills all the rest of the cell-space. The former do not play any further part of importance, but break up, the latter are the sex-cells, the cytoplasm of which now coalesces through a gap in the cell-walls, while their nuclei become closely apposed and ultimately unite (C, ♂ and ♀ k). From this union arises the fertilized spore, the so-called 'zygote' (D). The two small abortive cells so greatly resemble in their origin the polar cells of the animal ovum that it is difficult to resist the supposition that they bring about a reduction in the number of chromosomes. But the number of the chromosomes has not yet been determined either in them or in the sex-nuclei.