New research tools, especially cell fractionation and electron microscopy, opened for investigation the uncharted territory between biochemistry and cytology. The goal was to explain how cells carry out their basic functions. In the nineteenth and early twentieth century, the activity that had received the most attention was cell reproduction. The basic operations in cell division, including those carried out by chromosomes in the nucleus, had been described by cytologists using stains and the apochromatic lens decades before the advent of cell biology. The focus of early cell biology was rather on functions performed in the cytoplasm, especially capturing energy and synthesizing proteins. The first important steps in developing mechanistic explanations of these functions was to identify the mitochondrion and the endoplasmic reticulum as the cell organelles responsible for each, a project largely accomplished in the 1940s (although the name endoplasmic reticulum was not introduced until the early 1950s).