[The beginning of human development from the embryological viewpoint]

Human development commences with a fertilisation cascade, initiated when the haploid chromosomes of the oocyte and the sperm unite, thereby forming a programme for the creation of a human being. During the first few days development proceeds to implantation and placentation in only 30% of cases. The frailty of this early development shows that a human blastocyst, should it manage to emerge from the zygote, will only succeed in turning into a human being if there is a maternal contribution to implantation and placenta formation. The obstacles to early development represent basic genetic and epigenetic phenomena within the cytoplasm and in the genome of the blastomeres and are the results of the operation of natural laws the exact causal concatenations of which remain yet to be determined in detail by science. Philosophers, ethicists and lawyers seem to have difficulties in accepting that more research in this field is required. Finally, a look at the legal protection of the human embryo in Germany reveals that the earliest stage of human development prior to implantation is legally protected in vitro, whereas there is no such legal protection in vivo, owing to Germany's abortion law (StGB (German Penal Code) section 218), before implantation. Aspects of the research on human embryos and embryonic stem cells are here discussed from this point of view.