Establishing True Fertility in Hatching Eggs

Advice on egg candling from Marleen Boerjan of Pas Reform.
calendar icon 18 June 2011
clock icon 4 minute read

When it comes to discussions on fertility, two different definitions are practised. A true fertile egg contains a well developed germinal disc (blastoderm), which indicates that the oocyte, or zygote, was fertilised and an embryo developed during egg formation. Secondly, in the practice of the hatchery fertility is often based on candling, whereby all clear eggs are defined as unfertile and by default, the rest of the eggs are considered to be fertile. This second definition of fertility is strictly not correct since clear eggs may contain both truly infertile or they may contain (fertile) embryos that died early.

In hatchery practice, problems with fertility are usually first recognised during the candling procedure, when the number of clear eggs is higher than expected. To identify the time and the cause of embryonic death, the hatchery manager may perform an analysis of candled eggs. However, if candling is performed at transfer at day 18, as is often the case, it can be difficult to discriminate between true infertile eggs and eggs containing an embryo that has died before the blood ring stage. This is because membranes from dead embryos degenerate while the eggs are still in the incubator.

By candling between days seven and 10, it is possible to discriminate reliably between true infertility and early embryonic death for two reasons. Firstly, because embryonic membranes formed during the first days of incubation can still be recognised. Second, in clear eggs collected between days seven and 10, a change in the colour of yolk as a result of embryonic activity is clearly visible. The active young embryo transports water from albumen to yolk, which results in a whitish or light yellow ring around the embryo.

Figure 1a. Drawing showing the appearance of a fertile germinal disc

Figure 1b. Drawing showing the appearance of an infertile germinal disc

The fertile, unincubated egg contains an embryo (germinal disc or blastoderm) that developed from the fertilised oocyte (zygote) during egg formation in the oviduct. The oocyte is the female gamete that floats on the yolk. When the yolk is released in the oviduct, spermatozoa (male gametes) penetrate the yolk membrane, after which only one spermatozoon fuses with the oocyte to form the fertile zygote.

Finally, during egg formation in the oviduct, the zygote develops into the blastoderm, with a recognisable area pellucida (AP) surrounded by an area opaca (AO) (Figure 1a).

If, for whatever reason, the spermatozoa do not reach the oocyte, the egg remains infertile and the oocyte will degenerate to form nothing more than a small germinal disc. The infertile germinal disc is visible as a compact white spot with ruffled edges (Figure 1b). If hatching eggs are analyzed on arrival at the hatchery, before incubation, any issues with infertility can be communicated with the breeder farm without delay.


  • Candle eggs at transfer (day 18) as a standard routine.

  • If the number of clears is above acceptable or allowable standards, perform egg analysis to distinguish between infertility and early embryonic death.

  • Consider candling followed by egg analysis between days seven and 10 as a more reliable means of measuring true fertility.

  • Analyse a minimum of 10 fresh, un-incubated eggs on a regular basis when issues with fertility are suspected.

  • If true infertility is too high, communicate with the breeder farm about male and female management.

  • If the rate of early death before the blood ring stage is too high, evaluate conditions during storage and the transport of eggs – and ensure that the setter is bringing the eggs to incubation temperature rapidly and without interruption.

Drawings reproduced with kind permission of the publisher from Optimizing Chick Production in Broiler Breeders, by F.E. Robinson, G.M. Fasenko and R.A. Renema. Volume 1: Broiler breeder production series. Spotted Cow Press, Alberta, Canada.

June 2011
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