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What is the window during which division into twins is possible (for instance can it still happen after implantation in the uterus)? Is one twin the "original" and the other its clone?

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In the case of two healthy, viable identical twins, the separation generally occurs at the first division from one zygote to two cells. So they are equal partners in the split, not an original and a clone. During the first several cell divisions in the morula and blastula, there is no significant growth. So these divisions produce smaller and smaller cells. If one of these smaller cells is separated, it is very unlikely to be substantial enough to survive and implant in the uterus.

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