A SCANNING ELECTRON MICROSCOPE STUDY OF ASCIDIA MALACA EGG (TUNICATE). CHANGES IN THE CELL SURFACE MORPHOLOGY AT FERTILIZATION

¹ Institute of Zoology, University of Palermo, Via Archirafi 18, 90123—Palermo, Italy

Ascidia malaca eggs with and without envelopes were studied using the scanning electron microscope. Follicle cells, chorion, and test cells were examined and compared with those of other ascidian species. No appreciable differences were found. The surface topography of dechorionated eggs differed before and after fertilization.

The pole of the unfertilized egg was indicated by a small smooth region beneath the polar pit. The remaining surface of the egg was undulated. Short microvilli were scattered on the cell membrane except in the area nearest the polar pit. Surface displacements occurred in the fertilized egg, changing its morphological features, at various intervals after sperm penetration. The smooth region expanded shortly after fertilization, and the inferior part of the vegetal hemisphere was corrugated by pronounced undulations. Two new types of microvilli appeared. After the ejection of the first polar body the appearance of the egg surface changed: the animal hemisphere became corrugated, bearing numerous short microvilli, and the vegetal hemisphere showed slight undulations. At the vegetal pole microvilli concentrated to form a protuberance. After the ejection of the second polar body a diffusion of elongated microvilli was observed. The present results indicate that during ooplasmic segregation the movement of the cell membrane components produces changes in the surface topography. These govern the rearrangement of the cytoplasm.