The most important part in the elaboration of the germ-cells is the preparation of the nucleus, and this process is essentially the same both in the eggs and in the sperm. For most of the time the nucleus of
an ordinary cell consists of a bag made of the nuclear membrane filled with rather liquid protoplasm. When the cell is about to divide into two the nuclear membrane disappears, and out of the liquid contents there are built up a number of little solid lumps, which if the cell is killed can be stained very deeply with many dyes, and are therefore called chromosomes, from the Greek words for “ colour” and “ body.” Different chromosomes are often different in shape, so that they can be recognized, and it is very important to notice that they always occur in pairs, so that each cell has two of each kind. The number and shape of the chromosomes in the cell is fixed for any particular species, but is different in different species; some have as few as four, others up to one or two hundred. But as the chromosomes are always in pairs of similar ones the number must always be even. When the chromosomes become visible at the begin ning of an ordinary cell-division, each one is already split longitudinally into two half-chromosomes lying side by side. As the cell divides, these two halves separate from each other, and one half goes into each of the two cells which are formed. When the division is over they count as whole chromosomes, and gradually disappear into a normal fluid nucleus (Fig. 2).