STAGING AND AGING OF MAMMALIAN EMBRYOS AND FETUSES

The objective of this study was to elaborate a method of estimating th e age of embryos and fetuses of mammals of unknown ontogenetic age, ap plicable to both altricial and precocial mammals showing different dur ation of intrauterine development. The method consists of two steps: F irst, the embryo or fetus under study has to be ''staged''. It is meas ured, weighed and classified according to the characteristics given be low, in one of the 13 comparable ontogenetic stages. The second step i s ''ageing'': the staged embryo is placed in a point on the regression curve of the respective developmental stage that corresponds to the d uration of intrauterine development. The characteristics of the compar able developmental stages, the regression curves and the way in which they are calculated are presented. This method makes it possible to es timate the age of mammalian embryos and fetuses of unknown ontogenetic age, even of those already sectioned in histological series. In this way it is also possible to estimate the rates of morphogenetic events, the growth rate of the whole embryo or of its individual parts and/or organs.