The same is true for the forma­tion of the eye and probably in general. We have to consider the forma­tion of the various organs of the body as being due to the development of specific cells in definite loca­tions in the organisms which will grow out into definite organs no matter into which part of the organism they are transplanted. It is at present unknown what determines the forma­tion of these specific anlagen. They may lie dormant for a long time and then begin to grow at definite periods of development. We shall see later that we know more about the condi­tions which cause them to grow.

7. The fact that the egg, and probably every cell, has a definite structure should determine the limits of the divisibility of living matter. In most cases the complete destruc­tion of a cell means the cessa­tion of life phenomena. A brain or kidney which has been ground to a pulp is no longer able to perform its func­tions; yet we know that such pulps can still perform some of the characteristic chemical processes of the organ; e. g., the alcoholic fermenta­tion characteristic of yeast can be caused by the press juice from yeast; or characteristic oxida­tions can be induced by the ground pulp of organs. The ques­tion arises as to how far the divisibility of living matter can be carried without interfering with the total of its func­tions. Are the smallest particles of living matter which still exhibit all its func­tions of the order of magnitude of molecules and atoms, or are they of a different order? The first step toward obtaining an answer to this ques­tion was taken by Moritz Nussbaum,[139] who found that if an infusorian be divided into two pieces, one with and one without a nucleus, only the piece with a nucleus will continue to live and perform all the func­tions of self-preserva­tion and development which are characteristic of living organisms. This shows that at least two different structural elements, nucleus and cytoplasm, are needed for life. We can understand to a certain extent from this why an organ after being reduced to a pulp, in which the differentia­tion into nucleus and protoplasm is definitely and permanently lost, is unable to accomplish all its func­tions.[140]

The observa­tions of Nussbaum and those who repeated his experi­ments showed that although two different structures are required, not the whole mass of an infusorian is needed to maintain its life. The ques­tion then arose: How small a fraction of the original cell is required to permit the full maintenance of life? The writer tried to decide this ques­tion in the egg of the sea urchin. He had found a simple method by which the eggs of the sea urchin (Arbacia) can easily be divided into smaller fragments immediately after fertiliza­tion. When the egg is brought from five to ten minutes after fertiliza­tion (long before the first segmenta­tion occurs) into sea water which has been diluted by the addi­tion of equal parts of distilled water, the egg takes up water, swells, and causes the membrane to burst. Part of the protoplasm then flows out, in one egg more, in another less. If these eggs are afterward brought back into normal sea water those fragments which contain a nucleus begin to divide and develop.[141] It was found that the degree of development which such a fragment reaches is a func­tion of its mass; the smaller the piece, the sooner as a rule its development ceases. The smallest fragment which is capable of reaching the pluteus stage possesses the mass of about one-eighth of the whole egg. Boveri has since stated that it was about one twenty-seventh of the whole mass. Inasmuch as only the linear dimensions are directly measurable, a slight difference in measurement will cause a great discrepancy in the calcula­tion of the mass. Driesch’s results disagree with the statement of Boveri and support the observa­tion of the writer.

If we raise the ques­tion why such a limit exists in regard to the divisibility of living matter, it seems probable that only those fragments of an egg are capable of development into a pluteus which contain a sufficient amount of material of each of the three layers. If this be correct, it would certainly not suffice to mix the chemical constituents of the egg in order to produce a normal embryo; this would require besides the proper chemical substances a definite arrangement or structure of this material. The limits of divisibility of a cell seem therefore to depend upon its physical structure and must for this reason vary for different organisms and cells. The smallest piece of a sea-urchin egg that can reach the pluteus stage is still visible with the naked eye, and is therefore considerably larger than bacteria or many algæ, which also may be capable of further division.

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.

CHAPTER VII

REGENERATION

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.