Developmental stages of human embryos.

The embryonic period of human development was divided into eight main stages described as 1--unicellular, 2--blastomere stage, 3--blastoderm stage, 4--bilaminar stage, 5--trilaminar stage, 6--early somite stage, 7--stage of limb development, 8--late embryonic stage. These main groups were divided into further sub-groups. For description the decimal classification was used. The recommended classification is suitable for comparative purposes and was compared with hitherto used classifications of human embryos, in particular with Streeter's horizons.