I believe the answer is indeterminate embryonic cells. Determinate cleavage or the mosaic cleavage (cell division in early embryos) is in most protostomes. It results in the developmental fate of the cells being set early in the embryo development. A cell can only be indeterminate if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is a characteristic of deuterostomes- when the original cell in a deuterostome embryo divides, the two resulting cells can be separated and each one can individually develop into a whole organism.
