Cell Division Pattern during Gastrulation of the Ascidian, Halocynthia roretzi

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Abstract
With the aid of scanning electron microscopy, the spatial and temporal pattern of cell divisions during gastrulation of ascidian embryos was analysed and described up to the 306-cell stage. After the 64-cell stage asynchrony of cleavages became apparent, but the spatio-temporal cell-division pattern was highly invariant. The division pattern of cells in the animal hemisphere was regular, while that of cells in the vegetal hemisphere was complicated. The results obtained in the present study coincided with Conklin''s description (1905) with only a few discrepancies. Gastrulation started around the 110-cell stage and a late gastrula consisted of about 300 cells. In addition, the cell lineage of the neural tube was described.