NUCLEAR-CYTOPLASMIC RELATIONS IN THE MITOSIS OF SEA URCHIN EGGS. II. THE DIVISION TIMES OF WHOLE EGGS AND HAPLOID AND DIPLOID HALF-EGGS

¹ Departments of Radiology and Biology, Case Western Reserve University, Cleveland, Ohio 44106

Sea urchin eggs were cut into nucleate and anucleate halves. Following fertilization, the diploid half- and quarter-cells divided at the same time as whole cells, and the haploid half- and quarter-cells divided significantly later. However, there was no comparable increase in the duration of the second, third and fourth mitotic cycles of the haploid cells. Hence, the delay of the first mitosis of the haploid half-cells was attributed to the special consequences of the male pronucleus not fusing with a female pronucleus, and it was concluded that neither the cytoplasmic volume nor the ploidy of sea urchin eggs have significant effects on the timing of the mitotic cycle. Cutting experiments on eggs whose cytoplasmic organelles had been stratified by centrifugation suggested that some factor concentrated in the "mitochondrial layer" is essential for the maintenance of normal mitotic rates.