In the Monarch Incubator this required amount can readily be seen at once by the action of the regulating bar. It informs the operator just when he has enough,--when too much and when too little. Keep both lamps and chimneys clean, and have stated periods for turning your eggs, which should be done twice each day. As I said before, an egg-tester is not required with duck eggs, as they are so transparent that the whole process can be plainly seen without in the flame of a common kerosene lamp. If a duck egg is carefully examined, after being subjected to a heat of 102 degrees for twenty-four hours, a small dark spot will be seen about the size of a large pin-head. This little spot, if the egg is gradually turned, will always float over the upper surface of the egg. This is the life germ, and the first indication of fertility in the egg, and is represented in Figure 1. [Illustration: FIGURE 1.--SHOWING FIRST INDICATION OF FERTILITY.] At the end of forty-eight hours this dark spot will have nearly doubled its size, and a faint haze will appear around its edges a shade darker than the surrounding contents of the egg. This haze is the first appearance of the blood veins radiating out from the germ. Figure 2 shows how the egg appears at this stage with the air-cell slightly enlarged. [Illustration: 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. [Illustration: FIGURE 3.--EGG AT END OF 72 HOURS.] [Illustration: FIGURE 4.--EGG AT END OF 96 HOURS.]