The epithelial cells of the larval ileum of Culex pipiens increase in size but not in number during the larval stage and the first 10 hrs. of pupal life. The nuclei maintain the appearance of typical resting nuclei. In young first instar larvae the diam. of these nuclei ranges from 3-4 [mu], the same as that of the regenerative nuclei of the mid-gut which undergo mitosis and are clearly seen to have the specific number of 6 chromosomes. At the end of their growth period the nuclei of the larval epithelium are in the shape of flattened ellipsoids. The metamorphosis of the ileum takes place by division and multiplication of larval cells. At metaphase of the 1st division initiating metamorphosis multiple complexes of 48 and 96 chromosomes appear. The number of chromosomes is correlated with the size of the nucleus. The multiple complexes have apparently arisen during the larva] growth stage by successive division of all the chromosomes while in a diffuse or "resting stage" condition.