Figure 2.—Egg at End of 48 Hours.
At the end of the third day the dark spot, which is the heart of the embryo duck, can still be seen; but not so distinctly, because a dark circle some three-quarters of an inch in diameter will now appear in the upper surface of the egg, in the centre of which the dark spot is visible. This circle is several shades darker than the rest of the egg, and no matter how the egg is turned will always float in its upper surface.
Figure 3 represents the egg at this stage, with its enlarged air-cell.
Figure 4 represents the egg as it appears at the end of the fourth day. The circle surrounding and inclosing the germ will have nearly doubled in size, and is of a still darker hue; indeed, the whole contents of the egg is perceptibly darkened. If the egg is broken carefully at this date a delicate tracery of veins will be found to have enveloped the entire yolk of the egg, all originating from the centre or heart of the embryo; the pulsations of which (if the shell is removed) can now be plainly seen with the naked eye. This net work of veins cannot be plainly seen with a common lamp, but with a powerful glass are very distinct. This latter is not at all necessary in testing the egg.
