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The question of whether proteins necessary for cleavage are already present in unfertilized eggs or whether they are synthesized after fertilization has interested embryologists for a number of years. In 1959, Went (11) published immunological evidence for the presence in unfertilized sea urchin eggs of proteins also associated with partially purified mitotic apparatuses. Recently, Borisy and Taylor (1) have identified a colchicine-binding protein that appears to be a microtubule component which is present in unfertilized eggs as well as in isolated mitotic apparatuses. It is also true, however, that sea urchin eggs will not form spindles and undergo first cleavage if they are incubated with puromycin, an inhibitor of protein synthesis, from the time of fertilization (5). This fact has led several workers (4, 5) to postulate that the synthesis of at least some components of the mitotic apparatus might be necessary after fertilization. Radioautographs of fertilized sea urchin eggs incubated with radioactive amino acids reveal a greater labeling density in the spindle-forming region (4, 7), and considerable amino acid label has been found in isolated spindles (7, 10). A partial solution to the problem of whether postfertilization protein synthesis is necessary for cleavage in sea urchin eggs has been provided by experiments of Wilt, Sakai, and Mazia (12). Using improved methods for isolating purified mitotic apparatuses, they were able to demonstrate that, although some amino acid incorporation into the mitotic apparatus does occur normally after fertilization, this incorporation does not represent a

COMPLETION OF THE FIRST MEIOTIC DIVISION AFTER FERTILIZATION IN URECHIS CAUPO EGGS DESPITE INHIBITION OF PROTEIN SYNTHESIS