Program Studies on the Fundamental Effects of Radiation upon Aquatic Animals During the fiscal year of 1953-1954 work in other areas and in field-work planning necessitated the curtailment of a portion of this program. However, progress was made in completing the work initiated in previous years and on current projects. A. Biological effects of X-rays on invertebrates The major current research deals with the lethal effects of X-rays upon the embryological development of the freshwater snail, Helisoma. Documentary photomicrographs recording development of irradiated and control egg masses were reviewed and the results summarized for tabulation. Further irradia- tion experiments were conducted in an effort to reduce varia- bility or experimental error. Eggs undergoing the first cleavage were stained in order to relate the mitotic phases to certain characteristic stages in the microscopic appearance of the cleaving egg. During the summer a report of this experiment will be ready for publication. Early this fiscal year installation was completed on the beryllium-window X-ray unit, which expedited both radiobiological and roentgenographic experiments. Incidental to this activity a paper ‘© on the radiography of small fishes for meristic studies was published in August 1953.