Abstract
Earlier measurements of the surface behavior of cleaving seaurchin eggs (Mespilia globulus) haue been repeated an sand-dollar (Astrielypeus manni) eggs, extending through the second cleavage. The following points have been established.
1. The surface behavior of cleaving sand-dollar eggs is essentially similar, qualitatively and quantitatively, to that of sea-urchin eggs.
2. The thin hyaline plasma layer of Astrielypeius eggs does not mask the behavior of the endoplasmic surface as does the thick hyaline plasma layer of Mespilia eggs.
3. No difference can be detected between the surface behavior of the eggs at the animal pole, vegetal pole and equatorial region.
4. It is found from the distribution of particles that until the completion of cleavage, the original egg surface is stretched to cover the blastomeres. In eggs in calcium-free medium during interkinesis, a particle-free area appears in the furrow region, the extent of which corresponds numerically to the contact surface between the blastomeres in normal sea water. From these facts it is concluded that a new formation of surface takes place in the furrow region after cleavage has been completed, and further, that the contact area between the blastomeres in normal eggs is composed of this newly formed surface.
