Among vertebrates, oviparity, a condition in which young at least partially develop outside the mother's body and are supported by yolk, is the ancestral state to viviparity, the reproductive mode in which embryos are fully developed at birth. Viviparity in reptiles is found only in the squamates. Among the more than 100 origins of viviparity in squamates, many have occurred fairly recently and as a result some species contain populations of both reproductive modes, i.e., are reproductively bimodal. The evolution of viviparity is associated with many changes including an increase in oviductal egg retention times, a decrease in thickness of the eggshell and a decrease in secretion of the uterine shell glands that are responsible for eggshell secretion. The uterine morphology of the oviparous Saproscincus mustelinus and the reproductively bimodal Saiphos equalis were compared to study the uterine shell gland cycle.