UNEQUAL DIVISIONS AT THE 3RD-CLEAVAGE INCREASE THE NUMBER OF PRIMARY MESENCHYME CELLS IN SEA-URCHIN EMBRYOS

To clarify the distribution and behavior of the maternal factors that direct the differentiation of primary mesenchyme cells (PMC) in sea ur chin embryos, unequal division was induced at the third cleavage with the treatment of dinitro-phenol (DNP), and the numbers of differentiat ed PMC were examined. The most surprising finding was that the number of PMC was considerably increased in some of the DNP-treated embryos. This increase in the number of PMC was suggested to be closely related to the size of the precocious micromeres formed at the 8-cell stage. By measuring both the size of the precocious micromeres and the number oi PMC in individual embryos, it was suggested that almost all the de scendants of the precocious micromeres differentiated into PMC, ii the volume was less than 26 pL (about three times the volume oi normal mi cromeres). Cell tracing experiments ascertained that precocious microm eres with small volumes behave just like micromeres formed at the four th cleavage in normal embryos. The obtained results indicated that the maternal factors present in sea urchin embryos can direct, at least, more than three times the number of PMC, and that the number of cell d ivisions of the PMC lineage is not strictly regulated.