Secondary organs are those that arise from the primary ones, as the cœlom sacs, for instance, in the sea-urchin embryo. The primary organs are started by the setting free (“Auslösung”) of a new elementary process in the blastula, and later the secondary organs are started by new elementary processes that arise in the gastrula, which cannot appear until the gastrula stage itself is present as a starting-point. In other words, the elementary processes that are “given” in the egg can only come into action, or be set free after a certain stage has come into existence. This means that we must think of each organ that responds to a stimulus as having the possibility of receiving that stimulus, and also of answering to it. Even in inorganic nature every reaction must depend on a specific receptiveness and a specific answer. Driesch supposes that the receptivity is in the protoplasm, and the power to respond is in the nucleus of each cell. In this way he attempts to meet the difficulty that the nucleus is, in every cell, the bearer of the totality of all the “Anlagen”; but inasmuch as it is surrounded by a specific plasma, it is in a position to receive only certain stimuli, and can therefore only respond to certain causes. In the specific nature of the cytoplasm of the cell lies the prospective potence of every organ, and the possibilities of each cell are limited by its plasma; the cell becomes more and more limited as development proceeds. It may be said, therefore, that in the course of development the cells become actually limited in their possibilities, although they may still retain within themselves, in the nucleus, the potentialities of the entire organism. In the course of development each causal reaction brings about not only chemically specific differences, and thereby makes possible the introduction of new elementary processes, but the reaction also brings about by this very means a lessening of the possibilities of the cell, because each cell will now only respond to a more limited set of causes. We may say that the elementary process is not only the cause of the next change, but by virtue of its specific nature it is the beginning stage of the future reactions. Development proceeds from a few prearranged conditions, that are given in the structure of the egg, and these conditions, by reacting on each other, produce new conditions, and these may in turn react on the first ones, etc. With every effect there is at the same time a new cause, and the possibility of a new specific action, _i.e._ the development of a specific receiving station for stimuli. In this way there develops from the simple conditions existing in the egg the complicated form of the embryo.