INCUBATION 249 



the seventh day shows any considerable proportion of infertile, or 

 unhatchable, eggs, the good eggs remaining may be " doubled up " 

 and a part of the hens reset with the next lot. A second test is 

 usually made about the fourteenth day for the detection and removal 

 of dead germs. It is much more important that these should be re- 

 moved than that the infertile eggs should be taken away, for the 

 composition of the infertile egg is not changed during incubation, 

 while the egg containing a dead germ may rapidly decompose, is 

 more likely to be broken than an infertile egg or one with a live 

 germ, and, if broken in the nest, may spoil the hatch. 



The method of testing eggs in incubation is substantially the 

 same as the candling of market eggs, but the work is usually done 

 with a little more care. The ordinary incandescent electric light, 

 when convenient, makes a most satisfactory tester. An ordinary 

 hand lamp or lantern may be used, or if the place in which the 

 testing is to be done has a window toward the sun and can be com- 

 pletely darkened, the eggs may be tested by sunlight by placing 

 over this window a shutter, or thick curtain, having in it a hole of 

 suitable size (an inch in diameter, or a little larger), before which 

 the eggs may be passed. When an artificial light is used it may 

 be either placed in a small box with a suitable hole directly before 

 the light, or fitted with a metal chimney with a hole on one side.^ 

 The egg to be tested is held, large end up, at the hole before the 

 light. A strongly fertile egg at the seventh day will appear through 

 the tester as in Fig. 294. An infertile e.gg will be clear, but the 

 yolk may throw a light shadow. The apparent density of the egg 

 will usually be in proportion to the vitality of the germ, and those 

 in which at this time the shadow is relatively faint and the line of 

 the air cell not well defined will not usually hatch. Many poultry- 

 men leave these doubtful eggs until the second test ; but it is as 

 well to discard them at the first test, for the germ that does not 

 start well is not likely to produce a strong embryo. 



The average hatchable egg, tested with an ordinary light, shows 

 its development only by the increasing density of the shaded por- 

 tion, the enlargement of the air cell, and the sharper definition of 

 the line between the air cell and the growing embryo. Thin-shelled 

 eggs, or any eggs in very strong light, may show more of the detail 



1 See description, p. 171. 



