8. The most important fact which we gather from these data is that the cytoplasm of the unfertilized egg may be considered as the embryo in the rough and that the nucleus has apparently nothing to do with this predetermination. This must raise the question suggested already in the third chapter whether it might not be possible that the cytoplasm of the eggs is the carrier of the genus or even species heredity, while the Mendelian heredity which is determined by the nucleus adds only the finer details to the rough block. Such a possibility exists, and if it should turn out to be true we should come to the conclusion that the unity of the organism is not due to a putting together of a number of independent Mendelian characters according to a "pre-established plan," but to the fact that the organism in the rough existed already in the cytoplasm of the egg before the egg was fertilized. The influence of the hereditary Mendelian factors or genes consisted only in impressing the numerous details upon the rough block and in thus determining its variety and individuality; and this could be accomplished by substances circulating in the liquids of the body as we shall see in later chapters.
1. The action of the organism as a whole seems nowhere more pronounced than in the phenomena of regeneration, for it is the organism as a whole which represses the phenomena of regeneration in its parts, and it is the isolation of the part from the influence of the whole which sets in action the process of regeneration. The leaf of the Bermuda "life plant"--_Bryophyllum calycinum_--behaves like any other leaf as long as it is part of a healthy whole plant, while when isolated it gives rise to new plants. The power of so doing was possessed by the leaf while a part of the whole, and it was the "whole" which suppressed the formative forces in the leaf. When a piece is cut from the branch of a willow it forms roots near the lower end and shoots at the upper end, so that a tolerably presentable "whole" is restored. How does the "whole" prevent the basal end of the shoot from forming roots as long as it is part of the plant? A certain fresh-water flatworm has the mouth and pharynx in the middle of the body. When a piece is excised between the head and the pharynx a new head is formed at the oral end, a new tail at the opposite end, and in the middle of the remaining old tissue a new mouth and pharynx is formed. How does the "whole" suppress all this formative power in the part before the latter is isolated? It almost seems as if the isolation itself were the emancipation of the part from the tyranny of the whole. The explanation of this tyranny or of the correlation of the parts in the whole is to be found, however, in a different influence. The earlier botanists, Bonnet, Dutrochet, and especially Sachs,[142] pointed out that the phenomena of correlation are determined by the flow of sap in the body of a plant. These authors formulated the idea that the formation of new organs in the plant is determined by the existence of specific substances which are carried by the ascending or descending sap. Specific shoot-producing substances are carried to the apex, while specific root-producing substances are carried to the base of a plant. When a piece is cut from a branch of willow the root-forming substances must continue to flow to the basal end of the piece, and since their further progress is blocked there they induce the formation of roots at the basal end. Goebel[143] and de Vries have accepted this view and the writer made use of it in his first experiments on regeneration and heteromorphosis in animals.[144] At that time the idea of the existence of such specific organ-forming substances was received with some scepticism, but since then so many proofs for their existence have been obtained that the idea is no longer questioned. Such substances are known now under the name of "internal secretions" or "hormones"; their connection with the theory of Sachs was forgotten with the introduction of the new nomenclature.