A Guide to Terminology and Organization

Abstract

Age determination of embryos has always been associated with problems. Although all embryos are routinely measured and weighed, neither length nor weight permit an accurate estimation of age. To remedy this shortcoming, many embryologists have attempted to develop more reliable means. Mall from the Carnegie Institute started a staging system based on structural alterations occurring during embryonic development. Streeter [1] improved this system by dividing the embryonic period into 23 stages. Each stage is characterized by clearly defined and observable details of either the external form or the development of structures. Throughout the Atlas we have attempted to use Streeter’s stages as strictly as possible. Given the fact that our Atlas is concerned mostly with the later part of the embryonic period, we limit the short description to stages 15–23.