At this stage the nuclear membrane dissolves. And now each chromosome splits along its whole length into two exactly equal halves, which pass to opposite poles of the spindle. As all the halves do this at the same time, two groups of daughter chromo somes are constituted, each with a number of chromosomes exactly equal to that which the mother-nucleus had at the beginning of the division. Finally each group of chromosome halves gradually takes the form of a nucleus and the spindle disappears.
Nowadays we know fairly well what this complicated process called mitosis (see CYTOLOGY) really means. The chromosomes are protoplasmic bodies having an individual character, that is, they can only arise by division of other chromosomes exactly like themselves. Moreover in the so-called resting nucleus, i.e., the non-dividing nucleus, they carry on their individual existence. We know further, that they are the bearers of the hereditary proper ties of the animal. It is for this reason that they are so carefully divided and distributed to the daughter-nuclei at division. The spindle is the apparatus that organizes the exact separation.
Besides mitosis there is another kind of division called direct nuclear division or amitosis. It consists of a simple cleavage of the nucleus into two pieces and only appears in cells that are more or less abnormal or at any rate are incapable of giving rise to offspring with any chance of prolonged survival.
Earlier it was thought that amitosis was very widespread as the normal method of division in the Protozoa ; it was even thought that in contrast to mitosis it was the primitive, the original, type of nuclear division and that in some of the Protozoa the transi tional forms between mitosis and amitosis were preserved. Such is not the case ; in all groups of Protozoa we recognize to-day the occurrence of true mitosis, as complicated in every way and indeed often much more complicated than in multicellular animals.
As a matter of fact those processes which were thought to be tran sitional are nothing but mitoses whose true nature has been concealed by a variety of peculiar circum stances (fig. 9). In no case can we say that the method of nuclear division in the Protozoa is simpler or more primitive than in the higher animals and plants; the chromosomes of the Protozoa are no fewer than, and show in most cases the same peculiarities as, those of multicellular or ganisms, so that we may be permitted to regard them also as the bearers of the hereditary characters.
Most cases of true amitosis in the Pro tozoa are more or less pathological; only the so-called macronuclei of the Infusoria divide regularly by amitosis (fig. 8), and we shall see later with what exceptional condition this is associated.
Apart from these types of division there is one more, multiple nuclear division, where the nucleus breaks up at the same time into numerous nuclei ; this phenomenon is still very inadequately known. The so-called free nuclear formation has already been referred to.
In these cases we call the division budding or gemmation and the smaller animal we call a gemma or bud (fig. 12). Yet another method of multiplication is a "multiple division" in which the mother-animal divides at once into more than two—often several hundred—daughter-individuals (figs. 7, 13) ; sometimes the "mother" disappears entirely but sometimes a portion is left over, the so-called residual body, which finally degenerates. This process is preceded by a series of nuclear divisions (fig. 7).
The daughter-individuals arising by this process are rarely like the mother-individual. They are called, when provided with flagella, swarm-spores (fig. 12, i8b), and when surrounded by a stiff membrane and non-motile, spores; as a type these two forms are known as agametes. This unlikeness is also generally the case with gemmation and may occasionally arise from ordinary sim ple division. In all these cases, of course, the newly arisen smaller individual has to pass through some process of develop ment.
Requirements of Reproduction.—A protozoan reproduces itself only once, for the mother protozoan divides itself into its daughter products. Further the proto zoan, like the higher animals and plants, only reproduces when it is grown up and mature, having its special organization fully developed, and having reached its full specific size. When it arrives at this stage, division occurs automatically. The rate of multiplication, i.e., the time that must elapse before a protozoan just arisen from division can divide again. is very different in dif ferent species and depends, apart from many other things, on the food-supply and normal size of the species. Relatively small proto zoans that are well nourished are often mature in a few hours.