Abstract
Summary
1. Tadpoles of Rana pipiens and Xenopus laevis in premetamorphic stages were placed in solutions of thiourea up to 35 days. After 8 days or longer significant slowing of metamorphosis was achieved. Commonly complete arrest of metamorphosis occurred. 2. Compared to values in control animals, the sizes of cells and nuclei of the mesencephalic V nucleus were significantly reduced, and the nucleo-cytoplasmic ratio was increased, i.e., the changes were toward values characterizing younger larval stages. 3. Similar treatment of juvenile postmetamorphic Xenopus gave comparable but less marked changes in cell and nuclear sizes. The changes were greatest in smaller animals.
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