Loss of Egg Yolk Genes in Mammals and the Origin of Lactation and Placentation (image) PLOS Share Print E-Mail Caption The emergence of alternative nourishment resources (lactation and placentation) during mammalian evolution set the stage for mammals' progressive loss of egg yolk nourishment (as a consequence of the loss of egg yolk genes). Credit Image: Rasmus Kaessmann Usage Restrictions Attribution must be given to the photographer, Rasmus Kaessmann. Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.