Light micrograph of the results of a mitotic cell division in the human embryo, showing the 2 daughter cells produced. Cell division begins within hours of the zygote being produced by the fertilisation of the egg (ovum) by a single spermatozoon. The single cell of the zygote divides into 2, the 2 cells become 4, and so on. At this stage pregancy is not yet established; the zygote needs only to be in a fluid environment of progesterone (1 of the 2 main types of female steroid sex hormones) in order to divide. Progesterone also acts to transform the lining of the uterus to receive the fertilised egg.
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