Let us now turn to the different kinds of fertilization that occur amongst the Protozoa ; we find here a much greater diversity than amongst the higher animals and plants. The gametes of the Pro tozoa are not always so different from one another as the sperm and egg-cells of the higher animals. It is true that some types exist in which the female gametes look just like animal egg-cells; i.e., they are relatively large, stationary cells, rich in reserve ma terials. The male gametes in these species are constructed on the same plan as animal spermatozoa; they are relatively small, de ficient in protoplasm and are exceedingly mobile. We call the protozoans whose gametes differ in this way "oogamic," the fe rr ale gametes "macrogametes," the male "microgametes." But there are protozoans in which the pairs of gametes are absolutely indistinguishable; in these cases we speak of "isogamy" (fig. io). All imaginable transitional stages are to be found between isog amy and oogamy.
The point must be made clear that in the so-called vegetative phase individuals of most protozoans do not show whether they are male or female. Sex-determination itself, that "something" that makes the difference between a male and a female, is not really understood in most cases. In some species it has been shown that the distinction is the result of the "reduction-division" that follows fertilization. This reduction-division consists in the sepa ration of the two chromosomes, determining respectively maleness and femaleness, and their distribution to opposite cells which take the sex determined by these chromosomes. In those sexually dif ferentiated Protozoa in which the reduction-division takes place immediately before fertilization, sex-determination is probably of the same kind as in the higher animals (see SEx).
Two remarkable variations of merogamy are specially worth noticing. One is called autogamy. While as a rule only gam etes from different individuals are capable of fusing, in some Pro tozoa it happens that gametes that are the progeny of one indi vidual will fuse. When this occurs, as in Actinophrys, a helio zoan, where the vegetative individual first splits up into two gametes, one male, the other female, which then fuse, we call the process "paedogamy." When however the cell-division that fol loWs the so-called pregametic nuclear division is suppressed, and the whole sexual process really consists in a nucleus dividing and the two halves fusing straight away we speak of "autogamy." Of quite another kind from those just considered is the process called conjugation, found only in the infusorians and suctorians. Two individuals meet, unite at one point and exchange gamete nuclei. It must be remembered that the infusorians have nuclei of two kinds, large macronuclei and small micronuclei (fig. 4). In
the simplest case an infusorian has one nucleus of each kind. The two conjugating individuals (conjugants) unite first at their cyto stomes (fig. io). Then, as the next step, the micronucleus of each conjugant divides; the products of its division divide again and now three of the four new nuclei degenerate. The remaining nucleus divides itself again so that each conjugant has now two nuclei, offspring of its micronucleus. The macronucleus of each conjugant has meanwhile broken up and later disappears alto gether. And now out of each individual one nucleus, the "migrat ing nucleus," wanders into the other cell and fuses with the stationary nucleus there left behind. The two infusorians, now called "ex-conjugants" separate at this point and from the fusion product of the stationary and the migrating nuclei arise new micro- and macro-nuclei ; in the simplest form of the process the fusion nucleus simply divides and the two halves develop into micro- and macronucleus respectively.
In conjugation therefore it is not two cells that fuse to make one, but two hermaphrodite cells each of which forms a male, migrating, and a female, stationary, gamete-nucleus; the cells then exchange their male nuclei just as pairing snails or earth worms exchange their sperm. It is not hard to see why conjuga tion should rejuvenate these Protozoa, because the macro-nucleus degenerates and is re-formed from the fused gamete-nucleus; in doing this the organism dispenses with an old cell-constituent, the new one being supplied from the "potentially immortal" micro nucleus.
We have so far only concerned ourselves with the gametes of the Protozoa ; nothing more need be said about the process of their fusion, but what happens afterwards? The question is not at all easy to answer. The fate of the zygote amongst different species of Protozoa is very various. Only very seldom does the zygote turn directly into an ordinary vegetative individual, as is the case with the infusorians, Foraminifera (fig. 14) and myxo mycetes. Usually at least a short resting-stage is intercalated ; the cell surrounds itself with a stiff membrane and only later escapes from it to become again a vegetative individual; such is the case with the heliozoan, Actinophrys, and the Volvocineae. Very often however the zygote begins soon after fertilization to multiply in some special way. It breaks up by multiple division into a num ber of uninuclear cells which may surround themselves with mem branes and inside these divide further. Only from the germs formed in this way can new vegetative individuals arise.