Abstract
The addition of leupeptin to New cultures of rat embryos produces growth retardation and abnormalities of embryonic development. The effect is probably due to inhibition of the maternal protein breakdown necessary for embryonic growth. This function is carried out by the visceral layer of the yolk sac which shows distinct morphological changes akin to storage disease when leupeptin is added to the culture medium. We have not found it possible to reverse the effects of leupeptin by addition of amino acids to the culture medium.