Cell differentiation during early development of Nassarius reticulatus L. (Gastropoda Prosobranchia). I. Zygote to 16-cell stage.

The ultrastructure of the zygote and of early segmentation stages, up to the 16-cell stage, was investigated in normal Nassarius reticulatus from Roscoff (France). This study deals predominantly with structures that remain morphologically unchanged throughout this period, such as yolk granules, lipid droplets, and multimembranous vesicles. These organelles do not change in position, fine structure, or quantity from the egg to the 16-cell stage. The cortex and the vitelline layer also remain almost unchanged until the 16-cell stage. Cortical granules could not be observed and thus the thin vitelline layer is not transformed to a thick fertilization membrane. This phenomenon seems to be related to the facts that the eggs are protected by a tough capsule and that the cells of the embryo have to incorporate extraembryonic nutrient substances. From the 2-cell stage until the 16-cell stage micropinocytotic vesicles 500-2000 A in diameter are associated with the plasmalemma. Perhaps they help to incorporate the extraembryonic nutrient reserves. Desmosomes first appear at the 4-cell stage and are common at the 8-cell stage, when a small transitory blastocoel appears. Normally the cell borders are in close proximity during interphase. Syncytial connections between the blastomeres were not observed. Beginning at the trefoil stage the mitochondria increase in number and many apparent division stages are observed. The increase in mitochondria occurs in the perinuclear region throughout the embryo and does not result in a polar lobe especially rich or poor in mitochondria. From the 4-cell stage onwards, the number of ribosomes increases differentially in different blastomers.