Rat embryo (17.5 days). Close-up of a rat embryo at 17.5 days gestation. The embryo has been removed from its mother's uterus but is still attached to its placenta and remnants of its embryonic sac (at right). The developing eye, ear and nose are all recognisable on the head (at top). The legs are quite developed with individual digits visible. The gestation period of rats is 22-24 days with 6-12 young produced in a litter.