In the language of developmental biology, an “embryo” arises with the appearance, on the surface of the growing ball of cells created following conception, of a feature known as the “primitive streak”. In effect, this is the point at which the cells display the characteristics of a vertebrate. In humans, this happens at about 14 days following fertilisation. Before then, the developing ball of cells is known as a “blastocyst”. The Human Fertilisation & Embryology Act 1990 (the “Act”) made the word “embryo” include the blastocyst.   more »