Oocyte Competence to Maturation‐inducing Hormone. I. Breakdown of Germinal Vesicles of Small Oocytes in Starfish, Asterina pectinifera*

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Abstract
1-Methyladenine (1-MeAde) is the endogenous maturation-inducing substance (MIS) is starfish. However, small oocytes have no competence to 1-MeAde even at the concentration of 10-5 M. Furthermore, when they were injected with cytoplasm of fully-grown (large) and maturing (1-MeAde-treated) oocytes, known to contain maturation-promoting factor (MPF), they did not undergo germinal vesicle breakdown (GVBD). On the other hand, germinal vesicles (GV) of the small oocytes underwent nuclear breakdown when the small oocytes were fused with the large maturing oocytes. Therefore it is concluded that the GV of the small ooctyes are capable of undergoing nuclear breakdown in the presence of the sufficient MPF, but that the small oocytes can not amplify the injected MPF. Fused cells displayed particular shape changes during the course of nuclear breakdown of both the large and the small oocytes.