As far as I know the literature and the development of biological theories, the botanist Nägeli was the first to deduce, from the considerable difference in size between the egg-cell and the sperm-cell, the conclusion that the material basis on which the hereditary tendencies depend must be a minimal quantity of substance. The difference is especially great in animals, even in those species whose eggs may be called small, for instance, those of sea-urchins or of mammals; even in these the mass of spermatozoon is scarcely a thousandth part, often scarcely a hundred-thousandth part of the mass of the ovum. And yet the inheritance from the father and from the mother is equally great. Now as we know that vital powers have always a material basis, a minute quantity, such as is contained, for instance, in the spermatozoon of Man, must have implicitly in it all the hereditary tendencies of the father; and the conclusion is inevitable that in the ovum there can only be an equally minimal quantity of substance which is the bearer of the hereditary powers, for if there were a larger quantity of hereditary substance in the ovum its power of transmission would also be greater[16].
[16] The improbable assumption that the hereditary substance of the father may be in quality altogether different from that of the mother, and so may have the same power of transmission, and yet take up much less room, I leave out of the question altogether.
Fig. 69. Ovum of Sea-urchin (Toxopneustes lividus),
after E. B. Wilson, zk, cell-substance. k, nucleus
(so-called germinal vesicle). n, nucleolus (so-called
germinal spot). Below there is a spermatozoon of
the same animal (sp), magnified in the same proportion,
about 750 times.
Fig. 68. Diagram of a
spermatozoon. After E. B.
Wilson. sp, apex. n, nucleus.
c, centrosphere. m, middle
portion. ax, axial filament.
e, terminal filament.
If we inquire as to the part of the spermatozoon which bears this hereditary substance, we may exclude both the tail-thread and the middle piece (Fig. 68), the former because it obviously fulfils quite a specialized physiological function and is histologically adapted to this function, the latter because, from observation on the spermatozoon which has made its way into the ovum, we know that it contains the centrosome, the dividing apparatus of the nucleus. Thus there only remains the 'head' of the spermatozoon, which includes the nucleus, as the possible vehicle of the heritable substance. Therefore we are led to seek for the hereditary substance in the nucleus. But the hereditary substance cannot be a perishable substance which may at need be dissolved, in the literal sense of the word, and be formed anew; therefore we cannot look for it in the nuclear membrane, and just as little in the 'nuclear sap' which fills the meshes of the nuclear network, since the material on which heredity depends must necessarily be solid. Nägeli has clearly shown that we must assume a stable, that is, a solid molecular architecture. There thus remains only the nuclear reticulum with its chromatin granules, and when we remember what we have learnt of the behaviour of this chromatin substance during division and amphimixis we can entertain no doubt that the sought-for bearer of the inheritance is contained in the substance of the chromosomes.
The great care with which the chromosomes are halved by means of the complicated division apparatus led us earlier to regard them as a substance of complex and manifold qualities and of great physiological importance; their constant number in any one species, and the reduction of that number to half by means of the reducing divisions, justify us in concluding that they are permanent structures, physiological and morphological units, which undergo no more than an apparent irregular dispersion during the resting state of the nucleus. Finally, the fact that these supposed vehicles of inheritance occur in equal numbers in each of the conjugating germ-cells, and that this number is always, both in animals and in plants, half of the normal number occurring in somatic cells, is decisive. The logical necessity that the hereditary substance of both parents should be transmitted to the offspring in equal quantity could not be more precisely met than it is by the fact that half the normal number of chromosomes occurs in each of the sex-nuclei in the ovum. Personally, I have long been certain, on these grounds, that the chromosomes of the nucleus are the hereditary substance, and I expressed my conviction on this point almost simultaneously with Strasburger and O. Hertwig[17].
[17] More precisely, my conclusions were published several months later than those of the investigators named (1885). I think, however, that no one who is familiar with my writings for the years immediately preceding, which are collected in Aufsätzen über Vererbung und verwandte biologische Fragen (Jena, 1892), will dispute that the idea was reached by me independently. I attach importance to this because all my later work is based upon this idea.