The Effects of X-Radiation on the Early Cleavage Stages of the Snail, Ilyanassa obsoleta

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Abstract
The relative sensitivity of the various mitotic stages to X-radiation was determined by measuring the cleavage delay produced in the eggs of Ilyanassa obsoleta. Cleavage delay increases with the dose up to 4000 r when delivered 5 minutes after the start of pronuclear interphase. Above this dose there is a slight increase to 10,000 r, but as the dose increases above 5000 r the lethality before blastulation increases. Irradiation in telophase causes the greatest delay in the next cleavage. The elongation of the chromonemata is partially inhibited and probably interferes with the normal formation of the daughter chromosomes of the next mitosis. The second most sensitive stage is a short period in interphase. It is postulated to be a critical period in DNA synthesis. Little or no delay in the ensuing cleavage is caused by irradiation in stages from prophase to that cleavage, but the delay of the following cleavage increases, reaching its maximum when treatment occurs in telophase. The third cleavage shows a similar but smaller delay, indicating recovery. The mitotic stage increased most in duration is prophase. Interphase is also appreciable prolonged, but the other stages show little increase in duration. The delay is attributed to the difficulty of the separation of imperfectly formed chromatin strands in prophase.