“Laser confocal micrographs of stained chromatin showing superficial cleavage in a Drosophila embryo. The early nuclear divisions occur centrally. Numbers refer to the cell division cycle. At division cycle 10 (2 hours after fertilization), the pole cells form in the posterior, and the nuclei and their cytoplasmic islands (energids) migrate to the periphery of the cell. This creates the syncytial blastoderm. After cycle 13, the oocyte membranes ingress between the nuclei to form the cellular blastoderm.”
From Developmental Biology by Scott F. Gilbert; images courtesy of D. Daily and W. Sullivan.