University of Bristol
Wellcome Trust
Recommended by:
Society of Biology
PEEP for Physics & Ethics at GCSE

When does life begin?

Click to enlarge

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.

Zygote - Click to enlarge


Print out a copy of the picture and label as many of the following structures as you can: zona pellucida, polar body, perivitelline space, ovum cytoplasm, ovum surface membrane, male and female pronuclei, nucleoli.

Need help with egg anatomy?

After 18-21 hours, the first mitotic cell division of life is in prophase. The membranes of the pronuclei break down and the chromosomes condense and intermingle with one another. The spindle apparatus of metaphase appears and the zygotic chromosomes take up their position for the first time. Anaphase and telophase of the first mitosis of life result in syngamy, the final stage of fertilisation, in which the haploid chromosomes of the sperm and ovum have come together in a united diploid genome.

Question:  Question for discussion in class or on-line:

  • At what stage do you think a new life is formed?

 The developing embryo

What's your opinion?

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