An Ultrastructural Study of the Egg and Early Cleavage Stages ofLymnaea stagnalis, a Pulmonate Mollusc

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Abstract
Ultrastructural observations were made on the egg and early cleavage stages (2-, 4-, 8-, and 24-cell stages). An attempt was made to discern the nature of the ”subcortical accumulations“ (SCA) previously described at the light microscope level of resolution. A variety of organelles can be found at the periphery of the egg and embryos, but none of them are specific to this region. Although the morphological basis for the SCA could not be attributed to a particular morphological entity it is apparent that the SCA are not due to a specific large organelle (such as yolk platelets, lipid droplets, mitochondria) or a particular distribution or aggregation of such organelles. The SCA could be due to a cytochemical differentiation in the cytoplasmic matrix but it seems more likely that they reflect a differential accumulation of small organelles, possibly ribosomes.

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