Because mammalian embryos derive their nourishment from a placenta their early development is different from that manifested by embryos that develop outside the mother's body. Although development in mammals may be studied by using representative examples obtained by recovery methods, it is very fascinating to watch living embryos develop in the laboratory. Advances in culture techniques have made it possible to observe development of pre-implantation mammalian embryos entirely in vitro. The sequence of in vitro development may be interrupted at any time and selected specimens processed for scanning electron microscopy. Observation with the scanning electron microscope on developing mouse embryos revealed the mosaic nature of the oocyte and the dynamics of the egg membrane as egg activation proceeded from incorporation of the spermatozoon to extrusion of the polar body. During early cleavage the dividing cells remained attached to each other only by ionic attraction but prior to formation of the blastocyst the cells established extensive foci of junctional contacts. Interaction between blastocyst-stage embryos and uterine epithelial cells were approximated by co-culturing embryos with uterine cells and the SEM revealed the cell to cell interactions during peri-implantation development.