One point might still be mentioned since it may help to overcome a difficulty in visualizing the connection between the localization of a factor in the chromosome and the production of a comparatively large quantity of a specific chemical compound, _e. g._, a chromogen or a tyrosinase. We must remember that all the cells of an organism have identical chromosomes, so that a factor for an enzyme like tyrosinase is contained in every cell throughout the whole body. It is likely, however, that the same factor (which we may conceive to be a definite chemical compound) will find a different chemical substrate to work on in the cells of different organs of the body, since the different organs differ in their chemical composition. Thus it is conceivable that in the production of tyrosinase or of tyrosine not a single chromomere of one single cell is engaged, but the sum total of all these individual chromomeres of all the cells in one or several organs of the body. The writer has added this remark especially in consideration of the fact that some authors seem to feel that the chromosome conception of heredity is incompatible with a physicochemical view of this process. Since we have mentioned this difficulty which some writers seem to find in the chromosome theory of Mendelian heredity, it may be added that a single factor may suffice to determine a series of complicated reflexes. Thus the heliotropic reactions of animals are due to the presence of photosensitive substances, and it suffices for the hereditary transmission of the complicated purposeful reactions based on these tropisms that a factor for the formation of the photosensitive substance should exist.[215] [215] Loeb, J., “Egg Structure and the Heredity of Instincts,” _The Monist_, 1897, vii., 481. 5. Another point should be emphasized, namely that for Mendelian heredity it is immaterial whether the character is introduced by the spermatozoön or by the egg. This fact which Mendel himself already recognized is in full harmony with the conclusion that the chromosomes and not the cytoplasm are the bearers of Mendelian heredity, since only in respect to the chromosome constitution are egg and sperm alike, while they differ enormously in regard to the mass of protoplasm they carry. We can, therefore, be tolerably sure that wherever we deal with a hereditary factor which is determined by the egg alone the cytoplasm of the latter is partly or exclusively responsible for the result. We have already mentioned the fact that the rate of segmentation of the egg is such a character. Yet this character is as definite as any Mendelian character, and it would be as easy to discriminate two species of eggs by the time required from insemination to the beginning of cell division as it would be by any Mendelian character of their parents.