The conceptus and its parts: ontogenetic recapitulation in early human development.

Our understanding of the processes of human development that occur during and just after implantation is still incomplete. The anatomical studies by Erich Blechschmidt (1904-1992) at the University of Göttingen demonstrate the uniqueness and beauty of the early stages of individual human development or ontogeny. The interpretations of human embryology by Blechschmidt offer a simple unifying hypothesis: metabolic and biomechanical events are repeated, and this can be described as an ontogenetic recapitulation. This commentary provides a rationale for using some older terms and introducing new ones in the description of early human development. The product of conception is a conceptus; the outer part of the conceptus is the ectoblast and everything inside is the endoblast; the endocyst arises in the endoblast when the future amniotic fluid is forming. The human embryo arises from the innermost part of the endocyst. Terms such as morula, gastrula, and cyema, which are imported from zoology and ignore the role of the zona pellucida and constrained fluid compartments in the conceptus, can be avoided.