James Rankin, Natural and Artificial Duck Culture
“ This can be done much sooner, but a large machine full cannot be tested in a minute, and the eggs should be far enough advanced so that the operator can take two or three in his hand at once, and passing them before the flame, readily detect [Pg 60] the germ. I never use a tester for duck eggs, as a simple flame is sufficient, the egg being translucent.During the first stages of incubation the germ is very distinct, even at the third day. The clear eggs are reserved for family use or disposed of to bakers. An expert cannot distinguish them from a fresh-laid egg, either in taste or appearance. ”
