Comparison of the timing of pig preimplantation development, alterations in the ultrastructure of embryonic germ layers, and cytological changes of the uterine epithelial cells leads to the supposition that a close relationship exists between embryonic and uterine development during early pregnancy. The results of in-vitro studies of embryonic development and of experiments concerning asynchrony between embryos and uterine environment confirm this supposition, especially as far as the post-hatching period is concerned. It is suggested that successive steps in embryonic germ layer differentiation may induce specific developmental events and secretory activity of the embryos. A mutual influence of maternal and embryonic tissues appears to exist, but we can only speculate about the causes of many of the described phenomena.