Now if the complete hereditary substance of a germ-cell before the reducing divisions contains potentially all the primary constituents of the body, which it does as a matter of course, then it follows that after the reduction each germ-cell must either contain only half the primary constituents of the parents or all the primary constituents must be contained in the half number of chromosomes. The latter seems to me the only possible assumption, as I shall immediately proceed to show, and this is as much as to say that the primary constituents of at least two complete individuals must be contained in the chromosomes of the segmentation nucleus.
That this conclusion is correct is obvious from the fact that a whole, that is, a perfect individual with all its parts, develops from the ovum, and not a defective one. For suppose that each mature germ-cell contained only half the primary constituents of the body, it would be impossible that these halves should always exactly complete themselves to form a whole embryo when they are brought together in fertilization, after having been halved by mere chance during the preceding reducing division; it would be much more likely to happen that they did not complete themselves, and that their union would therefore result in an individual with certain parts wanting. If, for instance, in the sperm-cell only the anterior half of the body was potentially present, and this united with an ovum which likewise contained only the primary constituents of the anterior half, the embryo resulting from their union would lack the posterior half of the body, and so on. Of course so rough a division of the primary constituents is not to be thought of, but however fine we can imagine the halving of the mass of primary constituents to be, there would never be any guarantee that the two cells uniting in amphimixis would complete the mass of primary constituents again; indeed, the chance that the two exactly complementary halves of the mass would meet would rather become less the finer and more complex one imagines the halving by reducing divisions to be. A perfect embryo with all its parts would rarely arise, but now one group of parts, now another would be wanting, while another group might be developed double, or at least would be doubly present in the primary constituents.
But in addition to this the facts of inheritance show us that the resemblance to mother and father may express itself simultaneously in all the parts, or at least in the same parts of the child, as may be seen with especial clearness among plant-hybrids, and thus the conclusion is inevitable that even in the half number of chromosomes all the primary constituents of the whole body are present.
Let us go a generation further. If the species possess four chromosomes the child will have in its cells two maternal chromosomes (A) and two paternal chromosomes (B); what form will this proportion take in the germ-cells produced by the child? The maturation division can effect the reduction to two chromosomes in different ways; there may, for instance, be two paternal chromosomes (B) left in the one, and two maternal chromosomes (A) in the other daughter-cell, or one paternal (B) and one maternal (A) in the one, and a similar combination in the other cell. Let us follow the latter case further. A sperm-cell which contained the combination A and B might meet in amphimixis with an egg-cell of different origin also containing a similar combination of chromosomes, let us say a chromosome C from the mother, and a chromosome D from the father. We should then have in the segmentation nucleus of the fertilized ovum four different chromosomes, each of which contained the hereditary substance of one grandparent; we should have the four chromosomes, A, B, C, D, as the hereditary substance of the grandchild.
But since, as we have seen, the halved hereditary substance still contains the whole mass of primary constituents, each one of these chromosomes must contain the collective primary constituents of the whole body of the relevant grandparent[18]. The hereditary substance in the fertilized ovum thus consists of several complexes of primary constituents (chromosomes) each of which (an 'id') comprises within itself all the primary constituents of a complete individual.
[18] When I say the 'collective' primary constituents of the whole body of the grandparent this is not expressing it quite precisely, for, as we shall see later, each individual must arise from the co-operation of different chromosomes of different origin, not merely from one of the chromosomes contained in its germ-plasm. In the example given above, the body of each grandparent cannot have arisen only from a single chromosome, which was transmitted to his grandchild, but from the co-operation of this chromosome with three others, which have distributed themselves along other genealogical paths. But this does not affect the above chain of reasoning, for here it is not a question of whether all the primary constituents of the grandparent are present in the child—that can never be the case—but whether the primary constituents transmitted by him represent the whole body of an individual.
It can be made clear in yet another way that, as a consequence of sexual reproduction, the germ-plasm of each species must be composed of several 'ids,' individually different. Let us assume that there was as yet no amphimixis, and that we could look on at its introduction into the organic world; the hereditary substance of the beings which had previously lived and multiplied by division would consist of more or less numerous chromosomes similar to each other, so that, for instance, each individual would contain sixteen identical 'ids.' But if amphimixis were now to take place for the first time, in the same manner as it does to-day—that is, after the reduction of the number of the ids to half—in the first amphimixis eight paternal ids would unite with eight maternal ids to form the germ-plasm of the new individual, as is indicated in [Fig. 87] by a circle of spheres, of which ten are white and ten black as a sign of their difference. We may think of the figure as representing the 'equatorial plate' of a nuclear spindle with its ids arranged in a circle. Now, if two organisms of this generation, with two kinds of ids, unite in amphimixis after previous reduction of the ids, we have figure B, in which the paternal ids (pJ) are seen to the left of the line and the maternal ids (mJ) to the right, while each semicircle is in its turn made up of two kinds of ids, those of the grandparents (p2J and m2J, p2J1 and m2J1). The figures C and D show the two following generations, in which the number of identical ids is each time reduced to half, because eight strange ids are again mingled with them; in C only two ids are still identical, and in D all the ids are individually different, because they have come from different ancestors of the same species. Of course this would only be the case if inbreeding were excluded, because through it the ids of the same forefathers from two or more sides would meet; but prolonged inbreeding is a rare exception in free nature.
Fig. 87. Diagram to illustrate the operation of amphimixis on the composition of the germ-plasm out of diverse ancestral plasms or 'ids.' A-D, the ids of the germ-plasm of four successive generations: A, consisting of only two kinds of ids; B, of four; C, of eight; D, of sixteen kinds. pJ and mJ, paternal and maternal ids. p2J, grandpaternal; p3J, great-grandpaternal; p4J, great-great-grandpaternal ids. The marks in the ids themselves indicate their individually distinct characters.