Expressions of the prefertilization polar axis in sea urchin eggs

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The distribution of pigment granules in eggs of three species of sea urchins is described with reference to developmental stage and an egg's animal-vegetal axis of organization. Polarity in unfertilized sea urchin eggs has been a debated subject; present evidence demonstrates that the animal-vegetal axis is established before fertilization. The pigment pattern in some batches of Paracentrotus eggs exhibiting the celebrated “pigment band,” originally described by Theodor Boveri, is revised and is interpreted as a comparatively precocious expression of the underlying egg polarity. “Unbanded” Paracentrotus eggs and eggs of Arbacia lixula and Arbacia punctulata can be induced to exhibit the same pigment pattern by artificial activation. The induced pigment pattern aligns with an axis defined by polar bodies and the jelly canal, which are two external markers of the animal pole which are only rarely seen. It is therefore concluded that all of these eggs possess an animal-vegetal axis before fertilization even though it usually remains unexpressed until later developmental stages. Polarized changes in pigmentation are consistent with the following general mechanism: A change is triggered in the cortex of the vegetal pole; the change is programmed for a time which corresponds to the fourth mitotic division, even though mitosis itself is not involved; activation at fertilization initiates the “clock” in most cases, although in “banded” Paracentrotus eggs the “clock” is apparently started before ovulation; only the vegetal hemisphere's pigment is affected by the change. The nature of the underlying axis which defines animal and vegetal poles is discussed. Aspects of the axis have been tentatively traced back to the primary oocyte stage, but its fundamental nature remains unknown.