STAGING OF GASTRULATING MOUSE EMBRYOS BY MORPHOLOGICAL LANDMARKS IN THE DISSECTING MICROSCOPE

We describe and illustrate a set of morphological landmarks for classi fying mouse embryos by gross morphology from before gastrulation to th e beginning of organogenesis. These landmarks are visible at the resol ution of the dissecting microscope in embryos that are intact except f or reflexion of Reichert's membrane. Adoption of these criteria should facilitate interpretation of both the expression patterns of genes an d the consequences of experimental manipulation of embryos during earl y postimplantation development.