Color enhanced transmission electron micrograph of a human ovum, zygote cell stage, about 60 hours after fertilization. Immediately following the separation of chromosomes, the cell cleaves and becomes two cells called blastomeres. A nucleus can be seen in each. This occurs when a zygote undergoes its first in a series of mitotic divisions. Divisions will continue as the blastomeres move down the fallopian tubes to the uterine cavity. Magnification: x1000.