When does life begin?

Development of the Early Embryo

This diagram shows the events at the beginning of life, from fertilisation to the implantation of the embryo in the wall of the uterus.

Within four hours of the sperm head entering the ovum cytoplasm, each set of haploid chromosomes becomes surrounded by membranes, and are the known as the male and female pronuclei. The united ovum and sperm are now known as the zygote. The male pronucleus is slightly the larger, but both contain several prominent nucleoli. Gradually the two pronuclei, which can be seen as two distinct circles in the centre of the zygote in the picture, move to a central position within the cytoplasm of the zygote. At this stage the first cell division cycle of embryonic life begins.