Somatic Reduction Division in the Development of the Sea Urchin

Abstract

IN the sixteen-cell stage of the sea urchin egg, four micromeres occur around the vegetal pole. At the following division, which is delayed in the micro-meres¹, these cleave off ‘small micromeres’, which behave in many respects differently from the rest of the embryo². They do not flatten against their neighbours, appear optically empty in dark-field illumination, and cannot be vitally stained, for example, by nile blue sulphate. In all these respects the small micromeres resemble the polar bodies². This suggests a deeper similarity, namely, that the small micromeres function as ‘somatic polar bodies’, the division leading to their formation implying a reduction of the chromosome number.