Answer) Human embryo definition - Embryos are defined as whole human beings, at the early stage of their maturation. The term 'embryo', is similar to the term 'infant' and the term 'adolescent', which refers to a determinate and enduring organism at a particular stage of development.First, the zygote becomes a solid ball of many cells. Then it becomes a hollow ball of cells which is known as a blastocyst. Inside the uterus, the blastocyst implants in the wall of the uterus, this is where it develops into an embryo attached to a placenta and it is surrounded by fluid-filled membranes.