Notes: Summary 1. Histological analyses were made of imaginal discs and histoblasts during the larval development ofDrosophila melanogaster to determine the number of cells, the patterns of cell division and the growth dynamics in these adult primordia. Histological studies were also made of the imaginal rings which are the primordia of the adult salivary gland, fore-and hindgut, the anlage cells of the midgut and several larval and embryonic tissues. 2. In the newly-hatched larva, the immature eye-antenna, wing, haltere, leg and genital discs contain about 70, 38, 20, 36–45 and 64 cells respectively. These numbers include cells destined to form cuticular elements as well as peripodial, tracheal and nerve cells and probably the progenitors of adepithelial cells. The number of cells counted in the various imaginal disc anlagen is 1.5 to 4 times higher than the numbers deduced from genetic mosaic analyses by other investigators and reasons for these differences are given. 3. About 12 h after fertilization, mitosis ceases in all tissues of the embryo except the nervous system. After the larva hatches, mitosis resumes in most of the imaginal anlagen and in some larval tissues. The time of resumption of mitosis in the imaginal anlagen was determined after treating the larvae with colchicine for 2 h. 4. Among the imaginal discs, the eye disc is the first to begin cell division, at about 13–15 h after the hatching of the larva (first instar) followed by the wing (15–17 h), the haltere (18–20 h), the antenna, leg, and genitalia (24–26 h, early second instar), and finally the labial and dorsal prothoracic discs (52–54 h, early third instar). The cell doubling time for various discs was calculated from cell counts and the times agree closely with the doubling times deduced from clonal analyses by other workers: e.g., 7.5 h for the cells of the wing disc. 5. The imaginal ring of the hindgut first shows cell division early in the second instar. The imaginal rings of the foregut and salivary glands, the anlage cells of the midgut and the cells of the segmental lateral tracheal branches begin to divide early in the third instar. 6. The histoblasts which are the anlagen of the integument of the adult abdomen do not increase in number from the time of larval hatching until about 5 h after pupation when they begin to divide. Their behaviour contrasts with that of the histoblasts of the other dipterans such asCalliphora, Musca andDacus, which begin to divide during the second instar. 7. The histoblasts are an integral part of the larval abdominal epidermis and, unlike imaginal disc cells, secrete cuticle during larval life. Each hemisegment consists of an anterior dorsal, a posterior dorsal, and a ventral histoblast nest containing about 13, 6 and 12 cells respectively. The 62 histoblasts in each larval segment represent about 7–8% of the total number of cells that form the integument of that segment. 8. The number of cells in a particular type of histoblast nest was constant for both male and female larvae and among the different abdominal segments, except that the anterior dorsal group of the first and the seventh segments contains fewer cells than those of the other segments. Although the male and female adultDrosophila lack the first abdominal sternite and the male lacks the seventh abdominal tergite and sternite, the ventral histoblast nests of the first and the dorsal and ventral nests of the seventh abdominal segments are present in the larval stages as well as in the prepupa and have the same morphology and cell number as similar nests in the rest of the abdominal segments. 9. The cells of the imaginal discs increase in volume about six-fold and their nuclei increase in volume three-fold between the time of hatching and the initiation of mitosis. The histoblasts increase in volume about 60-fold and their nuclei increase in volume about 25-fold between larval hatching and pupariation. 10. Prior to each cell division, the nuclei of the columnar cells of the disc epithelium and of the histoblasts appear to migrate toward the apical surface of the epithelium. The cells round up and shift toward the apical region where mitosis occurs. After cytokinesis, the daughter cells move back to deeper positions in the epithelium. Because the nuclei of the non-dividing cells continue to lie deep in the epithelium, this intermitotic migration of nuclei gives these epithelia a pseudostratified appearance. 11. Analyses of the growth of larval cells and of organs confirmed the observations of earlier investigators that cell division occurs only in a few larval tissues, whereas growth in the rest of the larval tissues is by cell enlargement and polyteny. During larval life, cell division was detected only in the central nervous system, gonads, prothoracic glands, lymph glands and haemocytes. Each tissue began mitosis at a characteristic stage in larval life. The larval cells that did not divide, grew enormously, e.g., epidermal cells increased in volume 150-fold and their nuclei increased in volume 80-fold. 12. The adepithelial cells, which give rise to some of the imaginal muscles, were first identified between the thick side of the imaginal dise epithelium and the basement membrane at the beginning of the third larval instar (50–52 h). The origin of these precursors of mesodermal structures was analysed and evidence is presented that the adepithelial cells come from the disc epithelium. The question of the origin of the mesoderm of cyclorrhaphan Diptera is reviewed and it is suggested that the imaginal disc ectoderm may become segregated from the rest of the embryo before gastrulation has occurred, that is before the mesoderm has been established.