BEHAVIOR OF THE CELL SURFACE DURING CLEAVAGE

III. ON THE FORMATION OF NEW SURFACE IN THE EGGS OF STRONGYLOCENTROTUS PULCHERRIMUS

¹ From the Misaki Marine Biological Station, Misaki, Kanagawa-ken, Japan

1. In Strongylocentrotus pulcherrimus eggs, the granules seen in the living egg as orange-colored in white light and dark-red in light filtered through a Wratten No. 49 filter, and the large blackened granules seen in osmium-fixed sections, have been shown to be ideintical on the basis of size, position and behavior.

2. In unfertilized eggs, these granules are located directly beneath the surface, in which case it is difficult to distinguish them individually.

3. On fertilization, these granules migrate inward from the surface by 1.5 µ, leaving a finely granular zone of the same width at the cell periphery. This zone is called the "extra-granular zone" in the present paper.

4. During the process of segmentation, the equatorial surface of the uncleaved egg with the extra-granular zone and the granular layer forms the cleavage furrow with out change in the mutual relations of the component parts except for the fact that the extra-granular zone on the polar side becomes thinner and the granules become more numerous on the sides of the furrow.

5. After the completion of cleavage, the accumulation of granules in the furrow disappears and the extra-granular zone on the polar side regains its pre-cleavage width—i.e., the distribution of the granules around the blastomeres momentarily becomes uniform.

6. The above stage is immediately followed by a phase of formation of new surface along the furrow region in which the extra-granular zone retreats from the sides of the furrow and the granules come into direct contact with the protoplasmic surface film. This arrangement on the new surface is the same as that of the unfertilized egg.

7. Kaolin particles adhering to the cell surface at the head of the cleavage furrow later come to lie at the border between the old surface (provided with the extra-granular zone) and the new surface (which lacks this zone). This is taken as an indication that the new surface is formed from the tip of the furrow, and that the protoplasmic surface film which is carrying the kaolin particles does not slip over the underlying cortex.