Summar>. The developmeni of the ^inc-dcficient rai ombryo has been \tudicd in vitro using embryo cullure techniques. Normal 9-5 day embryov culiured for 4H h in ^crum obtained from /inc-deficient rais gre«' and developed to the same extent 3s those cultured in /inc-replcte serum, binbryos from dams which had been fed a /inc-del'icient diet since mating were also removed for culturi:. Such /inc-det'icient cnibryns tell iiitu two broad tnorpholnmicjl caicgorics. One group appeared identical to the normal embryos, \Uiile the others had appareinly normal visceral yolk sacs but small embryonic poles and retarded or abnormal embryonic development. Cultureof Ihe first group in either ;inc-deficient or replete serum produced morphologically normal embryos; however, those which appeared abnormal at day 9-5 were grossly malformed after 48 h incubation in either sera. When embryos were cultured in the presence of "Zinc, Ihe mosl severely affected zinc-deficient embryos accumulated as much ^inc as the zinc-replete and apparently unaffected ?inc deficient embryos, indicating that the malformations do not arise from an inability of ihe embryo or yolk sac lo accumulate /me from the surrounding fluid. The results from these studies suggest that the tirratogenic effects of ?inc deficiency cannot be induced by direct culiure of rinc-replcte embryos in /inc-deficient serutn. Furthermore, it would appear that maternal /inc deficienc) can exert its tcratological influence ptior to day 9-5 of scsiaiion and ihai these effects are npi readily reversible.

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