THE CARRIERS OF HEREDITY
Fig. 46.—Many different lines of study have made it seem probable that much, although not all, of the heredity of an animal or plant is carried in the nucleus of the germ-cell and that in this nucleus it is further located in little rods or threads which can be easily stained so as to become visible, and which have the name of chromosomes. In the above illustration four different views of the nucleus of the germ-cell of an earthworm are shown, with the chromosomes in different stages; in section 19 each chromosome is doubled up like a hairpin. Study of the fruit-fly Drosophila has made it seem probable not only that the hypothetical factors of heredity are located in the chromosomes, but that each factor has a perfectly definite location in its chromosome; and T. H. Morgan and his associates have worked out an ingenious method of measuring the distance from either end, at which the factor lies. Photomicrograph after Foot and Strobell.
Such is the essence of Mendelism; and the reader is probably ready to admit that it is not a simple matter, even when reduced to the simplest terms. To sum up, the principal features at the base of the hypothetical structure are these:
1. There exist relatively constant units in the germ-plasm.
2. There are two very distinct relationships which these units may show to each other. Two (or more) unit factors may be alternatives in the mechanism of inheritance, indicating that one is a variation (or loss) of the other; or they may be independent of each other in the mechanism of inheritance.
3. The mature germ-cell contains a single system of independent factors (one representative from each set of alternates).
The immature germ-cells, and body-cells, have double systems of independent factors (two from each set of alternatives).
4. The double system arises simply from the union of two single systems (i. e., two germ-cells), without union or even contamination of the factors involved.
In the formation of a single system (mature germ-cells) from a double (immature germ-cells), pairs of alternates separate, passing into different germ-cells. Factors not alternates may or may not separate—the distribution is largely a matter of chance.
Such are the fundamental principles of Mendelism; but on them was early grafted a theoretical structure due mainly to the German zoölogist, August Weismann. To understand his part in the story, we must advert to that much mooted and too often misunderstood problem furnished by the chromosomes. (See Fig. 46.) These little rods of easily stained material, which are found in every cell of the body, were picked out by Professor Weismann as the probable carriers of heredity. With remarkable acuteness, he predicted their behavior at cell-division, the intricate nature of which is usually the despair of every beginner in biology. When Mendelian breeding, in the early years of this century, showed temporary pairing and subsequent separation of units in the germ-cell, it was soon realized that the observed facts of breeding fitted to a nicety the observed facts (predicted by Weismann) of chromosome-behavior; for at each cell-division the chromosomes, too, pair and separate again. The observed behavior of transmitted characters in animals and plants followed, in so many cases, the observed behavior of the chromosomes, that many students found it almost impossible to believe that there was no connection between the two, and Dr. Weismann's prediction, that the chromosomes are the carriers of heredity, came to be looked on as a fact, by many biologists.