Introduction

Abstract

This discussion was arranged in the belief that diverse lines of research with microorganisms had contributed materially to the solution of the role of the cytoplasm in differentiation, and that the time was ripe to review the relationships and degree of accord between at least some of these newer lines of work. The treatment is from three points of view. First, there is the morphological framework as revealed by electron microscopy. Even though the genetic functions of the structures so revealed are as yet unknown, the fine-structure of the cytoplasm must ultimately be related to the expression of genetic functions. Secondly, there is the genetic determination of cytoplasmic variations, their nature and their stability and plasticity and the interdependence of nucleus and cytoplasm. Thirdly, there is a consideration of the biochemical self-sufficiency of the cytoplasm and its stability in terms of enzymes. It has often been stated that from the point of view of heritable variation, the cytoplasm plays a minor role. However, it plays the major role in expressing the characters determined by the genes, and its history and the effect of the environment upon it are not without influence upon the characters expressed, just as the genes influence one another. Thus the action of a gene, by which it is recognized, may be expressed in some cells or tissues, but apparently not in others. By orderly control of which genes appear to act at a given time, cells may be differentiated sequentially in a tissue for specialized functions.