For we must understand that, in the case of animals developed in eggs, they have the same relation to the yolk as the embryo of the vivipara has to the mother so long as it is within the mother (for since the nourishment of the embryo of the ovipara is not completed within the mother, the embryo takes part of it away from her); and their relation to the outermost membrane, the sanguineous one, is like that of the viviparous embryo to the uterus.