Fertile eggs, at the time of laying, cannot be told from infertile eggs, as the germ of the chick is microscopic in size.  If the egg is immediately cooled and held at a temperature below 70 degrees, the germ will not develop.  At a temperature of 103 degrees, the development of the chick proceeds most rapidly.  At this temperature the development is about as follows: 

Twelve hours incubation:  When broken in a saucer, the germ spot, visible upon all eggs, seems somewhat enlarged.  Looked at with a candle such an egg cannot be distinguished from a fresh egg.

Twenty-four hours:  The germ spot mottled and about the size of a dime.  This egg, if not too dark shelled, can readily be detected with the candle, the germ spot causing the yolk to appear considerably darker than the yolk of a fresh egg.  Such an egg is called a heavy egg or a floater.

Forty-eight hours:  By this time the opaque white membrane, which surrounds the germ, has spread well over the top of the yolk, and the egg is quite dark or heavy before the light.  Blood appears at about this period, but is difficult of detection by the candler, unless the germ dies and the blood ring sticks to the membrane of the egg.

Three days:  The blood ring is the prominent feature and is as large as a nickel.  The yolk behind the membrane has become watery.

Four days:  The body of the chick becomes readily visible, and prominent radiating blood vessels are seen.  The yolk is half covered with a water containing membrane.

These stages develop as given, occurring at a temperature of 103 degrees.  As the temperature is lowered the rate of chick development is retarded, but at any temperature above 70, this development will proceed far enough to cause serious injury to the quality of the eggs.

For commercial use eggs may be grouped in regard to heating as follows: 

(1) No heat shown.  Cannot be told at the candle from fresh eggs.

(2) Light floats.  First grade that can be separated by candling, corresponding to about twenty-four hours of incubation.  These are not objectionable to the average housewife.

(3) Heavy floats.  This group has no distinction from the former, except an exaggeration of the same feature.  These eggs are objectionable to the fastidious housewife, because of the appearing of the white and scummy looking allantois on the yolk.

(4) Blood rings.  Eggs in which blood has developed, extending to the period when the chick becomes visible. (5) Chicks visible to the candle.

The loss due to heated eggs is enormous; probably greater than that caused by any other source of loss to the egg trade.  The loss varies with the season of the year, and the climate.  In New England heat loss is to be considered as in the same class as loss from dirties and checks.  In Texas the egg business from the 15th of June until cool weather in the fall is practically dead.  People stop eating eggs at home and shipping out of the State nets the producer such small returns by the time the loss is allowed that, at the prices offered, it hardly pays the farmer to gather the eggs.  In the season of 1901 hatched chickens were commonly found in cases of market eggs, throughout the trans-Mississippi region, and eggs did well to net the shippers three cents per dozen.