Fig. 3

Gastrulation proceeds in two phases in L. mactans embryos. Tracings of external views of living embryos show that prospective cumulus cells (red and pink) internalize first at the blastopore (bp). Internalized cumulus cells are circled in red and migrate along red arrow. Other presumptive lower layer cells (shades of yellow) internalize later to form generalized mesendoderm. Note yellow cells at periphery of germ disc; these were traced and will internalize later. Blue cells are those known to persist in the superficial layer of the germ disc as long as their daughter cells can be visualized. Times in min. Scale bars 200 µm. Timelapse movies of Lm embryos 1 and 2 are shown in Additional file 4: Movie 1 and Additional file 5: Movie 2)