Introduction

Abstract

The mature vertebrate retina is a highly complicated array of several kinds of cells, capable of receiving light impulses, transforming them into neuronal membrane currents, and transmitting these in a meaningful way to central processing. Before it starts to develop, it is a small sheet of unconspicuous cells, which do not differ from other cells of the central nervous system. The chain of events which lead to the transformation from this stage into that of highly specialized cells ready to fulfil a specific task, is usually called “differentiation.” Originally, this word indicated firstly the process of divergence from other cells which were previously alike, and secondly, the change from an earlier stage of the same cell. It has become widespread practice to imply by the word “differentiation” also the acquisition of specific properties and capacities which are characteristic of a mature, i.e., specifically active, cell.