All of the facts reviewed in the foregoing pages converge, I think, to the conclusion drawn by Claude Bernard, that the nucleus is the formative centre of the cell in a chemical sense, and through this is the especial seat of the formative energy in a morphological sense. That the nucleus has such a significance in synthetic metabolism is proved by the fact that digestion and absorption of food, growth, and secretion cease with its removal from the cytoplasm, while destructive metabolism may long continue as manifested by the phenomena of irritability and contractility. It is indicated by the position and movements of the nucleus in relation to the food-supply and to the formation of specific cytoplasmic products. It harmonizes with the fact, now universally admitted, that active exchanges of material go on between nucleus and cytoplasm. The periodic changes of stainingcapacity undergone by the chromatin during the cycle of cell-life, taken in connection with the researches of physiological chemists on the chemical composition and staining-reactions of the nuclein-series, indicate that the substance known as nucleic acid plays a leading part in the constructive process. During the vegetative phase of the cell this substance appears to enter into combination with proteid or albuminous substance to form a nuclein. During its mitotic or reproductive phase the albumin is split off, leaving the substance of the chromosomes as nearly pure nucleic acid. When this is correlated with the fact that the sperm-nucleus, which brings with it the paternal heritage, likewise consists of nearly pure nucleic acid, the possibility is opened that this substance may be in a chemical sense not only the formative centre of the nucleus but also a primary factor in the constructive processes of the cytoplasm.
The role of the nucleus in constructive metabolism is intimately related with its rale in morphological synthesis and thus in inheritance ; for the recurrence of similar morphological characters must in the last analysis be due to the recurrence of corresponding forms of metabolic action of which they are the outward expression. That the nucleus is in fact a primary factor in morphological as well as chemical synthesis is demonstrated by experiments on unicellular plants and animals, which prove that the power of regenerating lost parts disappears with its removal, though the enucleated fragment may continue to live and move for a considerable period.
This fact establishes the presumption that the nucleus is, if not the actual seat of the formative energy, at least the controlling factor in that energy, and hence the controlling factor in inheritance. This presumption becomes a practical certainty when we turn to the facts of maturation, fertilization, and cell-division. All of these converge to the conclusion that the chromatin is the most essential element in development. In maturation the germ-nuclei are by an elaborate process prepared for the subsequent union of equivalent chro'matic elements from the two sexes. By fertilization these elements are brought together and by mitotic division distributed with exact equality to the embryonic cells. The result proves that the spermatozoon is as potent in inheritance as the ovum, though the latter contributes an amount of cytoplasm which is but an infinitesimal fraction of that supplied by the ovum. The centrosome, finally, is excluded from the process of inheritance, since it may be derived from one sex only.