TOTAL CELL NUMBER AND NUMBER OF THE PRIMARY MESENCHYME CELLS IN WHOLE, 1/2 AND 1/4 LARVAE OF CLYPEASTER JAPONICUS*

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Abstract
Total cell number and number of the primary cells of 1/2 and 1/4 larvae were counted at several developmental stages after hatching in comparison with those of a whole larva, using C. japonicus as material. To obtain partial larvae, blastomeres were isolated at the 2- or 4-cell stage in Ca-free sea water and cultured in natural sea water at around 23.degree. C. Isolated blastomeres cleaved as in situ, i.e., as part of an embryo. Although each partial embryo tended to spread into a plate, it acquired spherical shape prior to hatching of control whole embryo and developed normally in terms of both developmental rate and morphogenesis. Total cell number of a whole larva was .apprx. 620 just after hatching and increased almost linearly until it reached 1850 at the pluteus stage. A 1/2 and 1/4 larvae contained roughly 1/2 and 1/4, respectively, of the number of cells of whole larva through all stages counted. Numbers of the primary mesenchyme cells in the partial larvae tended to be slightly larger than a 1/2 or a 1/4 of that in whole larva. In whole larva, 35, 50, 56 and 58 were counted at the mesenchyme blastula, early gastrula, late gastrula and pluteus stage, respectively.