Genetical studies, centred as they have been on the relations between parent and sexual offspring, have thrown great emphasis on the nuclear control of heritable differences. True, other factors have been found of importance in particular cases. The effect of the mother especially on quantitative characters is, for example, a commonplace in mammals, and a number of instances are known in which transmission from parent to offspring is by way of the cytoplasm. These appear, however, more as exceptions springing from special circumstances, than as manifestations of any general principle; they constitute but small breaks in the smooth front of nuclear control. Thus, despite the occasional indication of cytoplasmic determinants showing continuity and permanence even when viewed on the genetical time-axis of generations, the nucleus emerges predominant in the long-term determination of differences among individuals. At the same time, the cytoplasm has shown itself to be of importance in a different way. It is the seat and agent of the action by which nuclear genes are recognized; and the not inconsiderable evidence available to us shows the genes to work through the agency of the cytoplasm even where their action is to control the behaviour of the nucleus itself (Mather 1948
b
). Indeed, the character of a cell in the immediate sense is the character of its cytoplasm, even though ultimately this in its turn may be the outcome of nuclear action (Mather 1948
a
). When, therefore, we turn to the relation of cells and tissues within an individual soma, and look at the determination of differences on the time axis, not of generations but of cell-heredity, the cytoplasm emerges as a key agent in the system. An understanding of its organization, structure and functioning is as essential as an understanding of the nucleus and the way it does its work. We can afford in fact to neglect neither; on our grasp of the essentially symbiotic relations between nucleus and cytoplasm, our appreciation of the parts they both play in the cell as a whole, depends our insight into development and differentiation. Our need for knowledge of the cytoplasm is the more obvious, because our ignorance of it in this connexion is the greater. Nevertheless, we should recognize that our knowledge of the nucleus has sprung largely from the study of long term heredity; nucleus and genes could well come to wear a different appearance when we turn to consider them in relation to cell variation within a soma.
Cell-to-cell variation sets a tissue-rheology–dependent bound on collective gradient sensing
