4. We are not concerned in this volume with the many applica­tions of the theory of heredity to the breeding of plants, animals, and man; the reader will find a discussion of these topics in the numerous writings of the special workers on genetics.[208] We are, however, interested in the bearing this work has on the concep­tion of the organism. Two ques­tions present themselves: Is the organism nothing but a mosaic of hereditary characters determined essentially by definite elements located in the chromo­somes; and if this be true, what makes a harmonious whole organism out of this kaleidoscopic assortment? We call it a kaleidoscopic assortment since a glance at the list of hereditary characters found in one chromo­some, according to Morgan, shows that there is apparently no physio­logical or chemical connec­tion between them, and second: How can a factor contained in the chromo­some determine a hereditary character of the organism? To the first ques­tion we venture to offer the answer which has been already suggested in various chapters of this book, that the cytoplasm of the egg is the future embryo in the rough; and that the factors of heredity in the sperm only act by impressing the details upon the rough block. This metaphor will receive a more definite meaning by the answer to the second ques­tion. The characters which follow Mendelian heredity are morpho­logical features as well as instincts. For the former we have already had occasion to show in previous chapters to what extent they depend upon the internal secre­tions or the existence of specific compounds in the circula­tion, and the same is true for the instincts (Chapters VIII and X). This then leads us to the sugges­tion that these determiners contained in the chromo­somes give rise each to the forma­tion of one or more specific substances which influence various parts of the body. We probably do not notice all the effects in each case, but when a special organ is affected in a conspicuous way, we connect the factor with this organ or the special feature of the organ which is altered, and speak of a determiner or factor for that organ, or for one of its characters. We also understand in this way why outside condi­tions should be able to overcome the hereditary tendency in certain cases, for instance why the influence of certain hereditary factors for pigmenta­tion should depend upon temperature as E. Baur observed.

The view, according to which the determiners in the chromo­somes only tend to give special characters to the embryo or to the adult while the cytoplasm of the egg may be considered the real embryo, receives some support from the fact that the first development of the egg is purely maternal, even if the egg nucleus has been replaced by sperm of a different species. If an egg of a sea urchin be cut into two pieces, one with and one without a nucleus, and the enucleated piece be fertilized with the sperm of a different species of sea urchin, the blastula and gastrula stages are purely maternal and only the skeleton of the pluteus stage begins to betray the influence of the foreign sperm inasmuch as this skeleton is purely paternal, according to Boveri. In all experi­ments on hybridiza­tion it has been found that the rate of cell division of the egg is a purely maternal character. Thus when fish eggs of a species, in which the rate of first segmenta­tion of the egg is about eight hours, are fertilized with sperm of a species for which the same process requires about thirty minutes or less at the same temperature, the rate of segmenta­tion is again about eight hours. There is then no chromo­some influence noticeable in the early development.

When two forms of sea urchins, Strongylo­centrotus franciscanus and purpuratus,[209] are crossed, certain features of the skeleton of the embryo, e. g., the so-called cross-bars, are a dominant, inasmuch as they are found in purpuratus and both the crosses, while they are absent in franciscanus. The development prior to the forma­tion of the skeleton is purely maternal. These observa­tions again lend support to the idea that the Mendelian factors of heredity must have the embryo to work on and that the organism is not to be considered a mere mosaic of Mendelian factors. This is further supported by the idea that the species specificity resides in the proteins of the unfertilized egg (see Chapter III), and it is quite likely that this species specificity decides which type of animal should arise from an egg.

The idea had been suggested that the factors which determine the future character might be ferments or enzymes, or substances from which such ferments develop. A. R. Moore[210] pointed out that the cross-bars in the skeleton of the hybrid between S. purpuratus and franciscanus develop more slowly than in the pure breed and that this should be expected if the determiners were enzymes. Since the pure purpuratus has two determiners for the development of the cross-bars (from both egg and sperm), the hybrids only one (from either egg or sperm), the pure purpuratus should have twice the enzyme mass of the hybrid. It is known that the velocity of a chemical reac­tion increases in propor­tion with the mass (or in some cases in propor­tion with the square root of the mass) of the enzyme; the cross-bars should therefore develop faster in the pure than in the hybrid breeds, as was observed by Moore. It was, however, not possible to obtain quantitative data.

On the other hand, it is obvious that this reasoning would not hold for all cases. Thus when beans with violet flowers are crossed with white-flowered beans the hybrids are pale blue, which indicates that the hybrids have less pigment than the pure violet. Now we know that the mass of enzyme does not influence the chemical equilibrium but only the velocity of the reac­tion. The hybrids and pure violets differ, however, in the mass of violet pigment formed, that is to say, in regard to the equilibrium. Hence the idea that the determiners are enzymes or give rise to enzymes is probably not applicable to cases of this type.

The experi­ments on the heredity of pigments are at present almost the only ones which can be used for an analysis of the chemical nature of the character and its possible determiner. The important work of G. Bertrand[211] and of Chodat[212] on the produc­tion of black pigment in the cells of animals and plants with the aid of enzymes has paved the way for such work. Bertrand has shown that tyrosine (p-oxy­phenyl­amino­propionic acid) is trans­formed into a black pigment by an enzyme tyrosinase which occurs in numerous organisms and is obviously the cause of pigment and coloura­tion in a great number of species. This discovery was utilized in the study of the heredity of pigments by Miss Durham, Gortner,[213] and very recently by Onslow.[214] The latter showed that from the skins of certain coloured rabbits and mice a peroxidase can be extracted which behaves like a tyrosinase toward tyrosine in the presence of hydrogen peroxide. This peroxidase was found in the skins of black agouti, chocolate and blue rabbits, but not in yellow or orange rabbits. The recessive whiteness in rabbits and mice according to this author is due to the lack of the peroxydase. There exists a dominant whiteness in the English rabbit which is due to a tyrosinase inhibitor which destroys the activity of the tyrosinase “and the dominant white bellies of yellow and agouti rabbits are due to the same cause.” “Varia­tions in coat colour are probably due to a quantitative rather than to a qualitative difference in the pigment present.”

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]

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.