A window to the embryo Mammalian embryonic development is a complex process, continuously changing in space and time. Q. Huang et al. designed an abdominal window to image mouse embryos in utero from embryonic day 9.5 to birth. Using this technique, they visualized dynamic activities during embryonic organ formation, including neurotransmission and cell division in the brain, autophagy in the retina, viral gene delivery, and placental drug transfer. They also tracked diverging fates of human and mouse neural crest cells in interspecies chimeras. Science , this issue p. 181