THE VITELLINE COAT OF THE MYTILUS EGG. I. NORMAL STRUCTURE AND EFFECT OF ACROSOMAL LYSIN

¹ Ochanomisu University, Tokyo, and The Misaki Marine Biological Station, Miura-Shi, Japan

1. Electron microscopy shows the egg of Mytilus edulis to be surrounded by a vitelline coat consisting of a layer about 0.5 µ thick, which corresponds to the refringent hyaline zone seen with the light microscope. This layer has a smooth inner surface, separated from the cytoplasmic surface proper by a space about 0.2 µ wide. The plasma membrane forms a brush of regularly arranged, straight microvilli 0.7-1 µ in length. These pass through and protrude slightly beyond the outer surface of the hyaline material, where their tips give rise to numerous extremely delicate fibrils which constitute at least one component of the "jelly layer." Fertilization does not cause any visible changes in these structures of the egg surface.

2. Exposure of a fertilized egg for one minute to a strong solution of acrosomal lysin causes an evident dissolution of the hyaline substance, and a two-minute exposure removes it almost completely, leaving the microvilli exposed but otherwise unaffected. The fibrils of the jelly layer also resist the lytic action. Exposure to lysin for 60 minutes induces no further changes in these structures.

3. It is concluded that the acrosomal lysin is specific for the single substance constituting the hyaline portion of the vitelline coat, and that the layer composed of this material is chiefly responsible for the configuration of the cleaving egg and the close contact of the blastomeres after cleavage.