SPECIES SPECIFIC PATTERN OF CILIOGENESIS IN DEVELOPING SEA URCHIN EMBRYOS

The events of cell division and ciliogenesis in individual blastomeres of developing embryos of the sea urchins Temnopleurus toreumaticus and Hemicentrotus pulcherrimus were followed with a Nomarski differential interference microscope. The number of cell divisions before initiation of ciliogenesis was determined with respect to species. In T. toreumaticus, ciliogenesis began about 4 hr after fertilization at 25°C. The sequence of ciliogenesis was as follows: cilia appeared first on smaller micromeres, followed in order by blastomeres derived from larger micromeres, those from mesomeres and finally those derived from macromeres. Blastomeres originating from mesomeres, macromeres, larger micromeres and smaller micromeres had completed the 8th, 9th, 7th and 5th divisions respectively, before they generated cilia. In H. pulcherrimus, embryos started to form cilia about 9 hr after fertilization at 18°C. Cilia appeared first on blastomeres of mesomere origin and, then on those of macromere origin. Before initiation of ciliogenesis, descendants of mesomeres and macromeres completed 9 and 10 rounds of cell division. Descendants of larger micromeres and the majority of cells derived from smaller micromeres did not acquired cilia even when the embryo began to rotate within the fertilization membrane. At this stage, the former had completed the 6th division and the latter the 8th division. Cell counts of blastomeres per embryo at the blastula stage also supported this observation.