Influence of Thiourea on Growth of Cells of Midbrain in Frogs.

Abstract

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. 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. Similar treatment of 14 juvenile postmetamorphic Xenopus gave comparable but less marked changes in cell and nuclear sizes. The changes were greatest in smaller animals. Apparently, in the larva, these large specialized cells depend continuously upon the amount of thyroid hor- mone in the blood for the degree of their differentiation. After metamorphosis this dependence appears to be decreased.