Fertilized eggs of Tripneustes esculentus, when exposed to concentrations of urethan which reversibly inhibited division or to lower concentrations which retarded division, rapidly accumulated urethan in concentrations equal to or exceeding the surrounding concentration. Unfertilized eggs accumulated urethan more slowly and did not exceed the surrounding concentration.

Most of the urethan accumulated in the eggs was in a free or loosely bound state. Cells inhibited by this free urethan began to divide again, once the urethan was removed by washing.

Using tracer technics, it was shown that fertilized and unfertilized eggs fixed only a very small portion of the carbonyl and methylene-carbon atoms from their accumulated urethan; no more than might be accounted for by fixation of carbon from urethan hydrolysis products, CO2 and ethyl alcohol.

Sea-cucumber sperm fixed more of the urethan carbon in proportion to carbon from labeled bicarbonate or ethanol than did eggs.

It is suggested that urethan is accumulated within the dividing cell and exerts its effect on mitosis while free or loosely bound within the cell. However, the specific fixation of carbon from two groups in the urethan molecule to sperm (high in desoxyribonucleoprotein) suggests that this compound combines with nuclear material. This affinity for nucleoproteins may be related to the well-known mutagenic, carcinogenic, and anti-cancer activity of urethan, since such processes are often considered to be associated with nucleoprotein metabolism.

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This work was supported by the American Cancer Society, upon recommendation of the Committee on Growth of the National Research Council, by the Research Fund of the Southern Research Institute, and by Mr. Ben May of Mobile, Alabama. The authors wish to express appreciation to the American Museum of Natural History for providing facilities for this work at the Lerner Marine Laboratory, Bimini, B.W.I.

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