BEHAVIOR OF THE CELL SURFACE DURING CLEAVAGE. VIII. ON THE CLEAVAGE OF MEDUSAN EGGS

¹ Misaki Marine Biological Station, Misaki, Kanagawa-ken and Zoology Department, Faculty of Science, Tokyo Imperial University, Tokyo

1. Characteristics of division involving an excentric mitotic figure: namely, astral rotation and spindle bending, are recognizable in the division of Spirocodon eggs.

2. Perforation experiments indicate that although the physical condition of the animal region of Spirocodon eggs is similar to that of sea-urchin eggs (Pseudocentrotus depressus), the vegetal region is quite fluid and deformable.

3. In Astriclypeus, the vegetal region is like the animal in physical consistency so that the vegetal region is pulled open (i.e., furrow formation takes place) sooner or later. In Spirocodon, the vegetal region is quite fluid. As a result, it is never pulled open but is simply drawn out until it finally breaks. This is the fundamental difference between heart-shaped cleavage and medusan cleavage.

4. Astral rotation is discussed in connection with cytoplasmic streaming, ectoplasmic movement and medusan embryology generally.

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