A comparison of the in vivo incorporation of S35 methionine by two‐celled mouse eggs and blastocysts

Five‐day‐old mouse blastocysts were transferred into the oviducts of recipients on the second day of pregnancy. S35 methionine was then injected into the recipients and the blastocysts and native 2‐celled eggs were recovered six hours later. Radioautographs reveal that the blastocysts incorporate S35 methionine while exposed to the tubal environment to the same degree that they would in the uterus. However, the 2‐celled eggs in the same oviducal environment incorporate little or no methionine. It is therefore concluded that the difference in the incorporation of S35 methionine is due to maturational changes in the blastocyst rather than to a deficiency of the labelled amino acid in the tubal lumen.