substances or parts originally present and by their reactions to external stimuli new substances and parts appear which had no previous existence, just as new substances result from chemical reactions. This is "creative synthesis" in philosophy, epigenesis in development. Differentiations appear chiefly in the cytoplasm, but only as the result of interaction between cytoplasm and nucleus. Similarly, it may be argued, smaller units of organization, such as chromosomes or chromosomeres, do not in themselves give rise to any adult part, but only as they interact upon one another are new parts formed.
In many cases the first formation of such new substances appears in the immediate vicinity of the nucleus and, like assimilation itself, is evidently brought about by the interaction of nucleus and cytoplasm. In certain cases it can be seen that the achromatin and oxychromatin which escape from the nucleus during division take part in the formation of new substances in the cell body, and since the oxychromatin is derived from the chromosomes of the previous cell division, it is probable that the chromosomes are a factor in this process.
Weismann maintained that the chromosomes and the inheritance units contained in them undergo differentiation by a process of disintegration and that these disintegrated units escape into the cell body and there produce different kinds of cytoplasm in different cells. A somewhat similar view was advanced by de Vries in his theory of intracellular pangenesis. However, as we have seen already, there is good evidence that the chromosomes do not undergo progressive differentiation in the course of development; they always divide with exact equality, and even in highly differentiated tissue cells their number and form remain as in embryonic cells.
On the other hand, the cytoplasm undergoes progressive differentiation, and when by pressure or centrifugal force such differentiated cytoplasm is brought into relations with strange nuclei the differentiations of the cytoplasm are not altered thereby, thus showing that the different nuclei are essentially alike and that differentiations are mainly limited to the cytoplasm. Thus the differentiations of cells are not due to the differentiations of their nuclei, but rather the reverse is true,—such differentiations of nuclei as occur are due to differentiations of cytoplasm in which they lie. Nevertheless, differentiations do not take place in the absence of nuclear material, and it seems probable that the interaction of nucleus and cytoplasm are necessary to the formation of the new cytoplasmic substances which appear in the course of development.
2. Segregation and Isolation of Different Substances in Cells
But differentiation consists not only in the formation of different kinds of substances in cells, but also in the separation of these substances from one another. This separation is brought about to a great extent
