WHY tissues and cells are differentially susceptible to radiation is one of the fundamental questions in radiology. It is a remarkable fact that some cells can withstand very large doses of radiation while others succomb to very small doses. For example, Crowther (1) has shown that around 80,000 roentgens of x-rays are required to inactivate a certain protozoan (Colpidium), whereas Scott (2) has found that cells of the chick allantois are destroyed by two or three roentgens. Although the tissues of the human body probably do not manifest such wide differences in radiosensitivity, they do nevertheless differ considerably and it is largely upon this condition that the beneficial results obtained in radiotherapy depend. In an earlier report (3) we have shown that the radiosensitivity of Drosophila melanogaster (fruit fly) eggs changes abruptly and over wide ranges during early development, and have also pointed out the bearing such changes have on quantitative radiobiological results. Since the nature of developmental activity is different at different stages it has been possible also to correlate changes in radiosensitivity with changes in biologic activity and thus ascertain what conditions in the organism are associated with radiosensitivity. Such procedure has yielded information concerning the factors which determine susceptibility to radiation. Therefore, we wish at this time, to take up in some detail the biological conditions at the different stages and show their relation to radiosensitivity. Early Development in Drosophila Eggs.—The mature Drosophila egg consists of a cell wall or vitelline membrane within which are the single egg nucleus and many yolk granules suspended in a semi-fluid protoplasm (Fig. 1-A). Surrounding the egg is the egg case or chorion which is secreted by ovarian cells and which is lost at the time of hatching. As the eggs pass through the vagina of the female fly they are capable of being fertilized by spermatozoa which have been stored by the male in the seminal receptical. Sperm enter the egg and one sperm nucleus unites with the egg nucleus to form the segmentation nucleus. The segmentation nucleus, lying free among the yolk granules, is surrounded by a small volume of oöplasm and gives rise to other nuclei by mitosis. Nuclei only are involved in the early cleavages, cell membranes not being present. The cleavages are synchronous among the cells and take place at the rate of one every 10 to 12 minutes at room temperature, 22° to 25° C. At the 256 cell stage (eighth cleavage) the nuclei begin to migrate to the periphery where cell membranes are formed around them and where they become arranged in a single cellular layer to form the blastoderm (Fig. 1-B).