Determination and its fixation

Abstract

Differentiation consist of the orderly development of cells and cell-lineages along different but mutually adjusted paths, and once started along a particular path a cell-lineage gives every appearance of being inexorably bound to follow it out to the end. The capacity to vary from the path diminishes progressively, and often rapidly, until, as animal tissue culture shows us, the lineage may become so fixed in its properties that no known experimental means will bring about a change beyond temporary modifications directly dependent on the external conditions applied to the cells and vanishing as soon as these conditions are removed. When the developmental course has been set behaviour of the cells is said to be determined and determination may take place in a lineage at a very early stage in ontogeny. Indeed in those animals having mosaic eggs, the basis of determination is already present in the egg itself as a zoning of the cytoplasm. Furthermore, the determination may occur rapidly over a narrowly defined and relatively fugitive stage in development, as is testified by the ability of physical conditions or chemical and biochemical action to alter the course of development in particular ways only if applied at certain times, which may then come to be known as the effective periods. How is it that a cell or cell-lineage, at one time labile enough in its properties to be capable of change by determinative agencies, can become so restricted in its capacity for change from the determined path?