Definitions of Common Hatchery Terms
Blastoderm without embryo (BWE) - when candled this egg shows a blood ring. On break-out there are no visible embryo structures.
Blood ring - this usually refers to the candled appearance of an egg in which the embryo has died at a young age.
Candled fertility - the percentage of clears removed by candling; this is not true fertility.
Cystic embryo - broken-out appearance is similar to a BWE except that embryo tissue is visible. This is a type of early dead.
Dry bulb temperature - the temperature measured with a standard thermometer or electronic sensor.
Fertile, no development (FND) - a rarely diagnosed condition in which the blastodisk was fertilized but died before the egg was laid or before growth could be initiated in the incubator.
Fertile, pre-ovipositional death - a rarely diagnosed condition characterized by a blastodisk which appears fertile but died before the egg was laid by the hen.
True fertility - the percentage of hatching eggs which are fertile; this can only be determined by incubation, candling and breakout of the clears to determine which eggs were fertile or by breaking out potentially fertile eggs to examine the germinal disc (e.g. a sample might be examined to estimate fertility of a flock).
Hatch of fertiles or Hatchability - the percentage of fertile eggs which hatch (hopefully above 85%).
Hatcher - a machine used to maintain proper conditions for embryos during the final few (usually three) days before hatching.
Incubator - a machine used to maintain proper conditions for setting avian eggs.
Hatch, Percent hatch, Hatching percent, or Hatch of total - the percentage of all eggs set which hatch whether they were fertile or not (a typical hatch might be 80% to 90%).
Malpositions - an embryo in any position except head under right wing positioned in the large end of the shell. Examples of malpositions are head under left wing or head between legs.
Pip - an egg in which the chick has broken the shell in an attempt to hatch. Sometimes chicks die shortly after piping the shell.
Positive development (PD) - these eggs are candled out as clears because there is no blood formation. The germ was fertile but died soon after cell growth resumed when the egg was warmed above 80oF.
Pre-ovipositional death - embryo dies before the egg is laid.
Relative humidity - is a measure of the water vapor or moisture in air. It can be determined from the wet-bulb and dry-bulb temperature using a psychometric chart. Incubation instructions are sometimes give in wet-bulb temperature or relative humidity.
Spread - the difference between Fertility and Hatch (a 10% to 12% spread is typical for chicken eggs).
Wet bulb temperature - the temperature measured by a standard thermometer equipped with a wet sock over the bulb. For accurate measurements air must be moving over the wet sock to provide evaporation. A sling psychrometer can be used to measure wet and dry-bulb temperature. Electronic sensors are now available to measure humidity of air in incubators and egg storage rooms.