Produced from a single cell, the embryo develops through cellular division (mitosis). In animals, embryogenesis refers to the transformation of the egg into an embryo. The first embryonic development stages are called morula (8-16 cells), blastula, gastrula, neurula. Among humans, the term foetus is used starting from the third month.
The subject of human embryos raises many ethical issues. As a potential person from the time of conception, the point at which an embryo takes on the status of a human person is chiefly a religious and philosophical question. There are contrasting opinions and attitudes regarding abortion or limits that may be imposed on the manipulation of embryos, whether for research or for medical purposes.