Xenopus embryo staging In order to repeat a published experiment, or have someone else repeat yours, it is important to use the same materials. For developmental studies, that means knowing the precise point in development that the embryos have reached. Embryos develop at different rates depending on the temperature of incubation, so the time of incubation gives only an approximation of the developmental age. For this reason, staging series based on morphologic criteria have been prepared. The following staging series has been prepared based on Nieuwkoop, P.D. and Faber, J. 1956. Normal Table of Xenopus laevis (Daudin). North Holland Publishing Co., Amsterdam.