Every cell in our bodies has its nucleus with its equipment of chromosomes. An interesting fact already spoken of in Chapter V is that according to our best knowledge the chromosomes in any cell of any one of us are exactly like those in all the other cells of the same person. The chromosomes in the muscle cells are exactly like those in the nerve cells, and both correspond exactly with the chromosomes of the ordinary cells in the germinal tissue. In every one of these cells the chromosomes are arranged in pairs. It happens that in human beings the number of pairs is twenty-four, with one pair incomplete in some of us for a reason that will be explained presently; this is an awkwardly large number for our present purpose, since we have allotted only fifty-two determiners altogether, so we shall do some more pretending and set the number of pairs at nine. One more change will have to be made from the real state of affairs before we can go on with the description; this is to suppose that our various bodily features can show only one difference, instead of the many of which they are really capable. For example, we shall suppose that the hair can be either black or light, but never red; the nose can be Roman or Greek, but never Irish. By making this supposition, we can let the large letters stand for one set of hereditary determiners and the small for the same features but with exactly contrary traits. According to this arrangement, if we had two persons side by side one of whom had only large-letter determiners in his chromosomes and the other only small, they would have the same general human make-up, but in every possible detail one would be the exact opposite of the other. It has been proven by complicated studies which we cannot take time to describe that the determiners are grouped in the different chromosomes in a definite plan. We saw a moment ago that the chromosomes are in pairs. This pairing is an essential part of the arrangement, for every hereditary bodily feature actually has two determiners governing it, which lie in corresponding positions in the two chromosomes of the pair. To illustrate how this works out, let us suppose that the chromosomes of the first pair contain determiners A, B, and C. Each of these three determiners will be present in both members of pair number one, and if this is true of any cell it will be true of all the cells, and in any other human being either they or the corresponding small-letter determiners, a, b, and c, will occupy pair one of the chromosomes. Furthermore, they will lie in a row within the chromosomes, always in the same order; thus if A and C are at the ends with B in the middle in one chromosome, every other chromosome that contains these three determiners will have them in the same order. [Illustration: DIAGRAM ILLUSTRATING VARIOUS ARRANGEMENTS OF DETERMINERS IN A SINGLE CHROMOSOME PAIR 1 and 2, Pure; 3, Hybrid.]