It’s time to extend the 14-day limit for embryo research | John Harris

A switch to a 21-day deadline would allow scientists to better understand miscarriages and the possibilities of using stem cells to treat diseases

The 14-day rule for permissible embryo research was conceived by the philosopher Mary Warnock as an answer to the question, always raised in relation to controversial science, of "where to draw the line".

The somewhat curious reason for this cut-off was that the individual soul could not be bestowed by God on an individual who might turn out to be two individuals. Fourteen days was the point at which an embryo could no longer split into two identical twins, requiring two souls and not just one soul between them. This was said to be the point at which the individuality of an embryo is assured, because it can no longer become two people.