There is often uncertainly in determining fertility when freshly laid hatching eggs are examined. Detrimental and costly management decisions can result if fertility is incorrectly assessed. Determination of fertility prior to incubation is often difficult and a person may not always be 100% correct, but with an understanding of early embryonic development one can be more precise.

Ovulation and Early Embryonic Development
After ovulation of the yolk follicle (ovum) begins the descent through the reproductive tract (oviduct) of the hen. Fertilization occurs shortly after ovulation in the infundibulum of the oviduct before any albumen is placed around the yolk. At the area on the yolk where sperm enter and fertilize the hen's component of the egg is called the germinal disc. The germinal disc provides the hen's genetic contribution to the embryo just as the sperm provides the rooster's genetic contribution. If the germinal disc is fertilized the disc (now an embryo) is called a blastoderm. We often think that embryonic development does not begin until the egg is placed in the incubator, but this is far from reality. Embryonic development actually occurs while the follicle is traveling through the oviduct of the hen whose body temperature is approximately 41oC (105.8oF). Development begins immediately after fertilization. Since it takes about 24-26 hours for the egg to be completely formed the blastoderm has already been developing for about one day. When the hatching egg is placed in the egg cooler at the breeder farm, the temperature is below that required for embryonic development to continue, and therefore, further embryonic growth ceases.

Variation in Germinal Disc Appearance at the Time of Lay
Because we are dealing with the growth of an animal there is natural biological variation in the stage of embryonic development at the time of lay. This variation may occur between individual hens, between different bird strains, and/or between flocks of different ages. In industry it is common for the germinal disc of a fertile egg to be characterized as having a a donut appearance. This appearance holds true for many blastoderms, but not all. Chicken blastoderms at different stages of development at the time of lay look different when examined macroscopically (with the naked eye).

Categorizing Early Embryonic Development
Researchers have categorized embryonic development into stages based on the appearance of the blastoderms under a microscope. Most freshly laid eggs have embryos that are at Stage X of development; however, some eggs may be laid at less developed Stages VIII or IX, while other fresh eggs may have embryos at more developed Stages XI or XII.

Structure of an Embryo Prior to Incubation
At the time of lay, the blastoderm is composed of approximately 60,000 cells that sit underneath the yolk membrane on top of the yolk. Only the cells of the blastoderm of the egg divide and develop into the embryo; the large amount of yolk on which the blastoderm sits is there only as a food source for the growing embryo.

The embryo (or blastoderm) is approximately 1/4 inch (3 mm) in diameter and has a round, uniform appearance. Between the interior portion of the circular mass of developing embryonic cells and the yolk is a small cavity called the subblastodermal cavity. The different appearances of the blastoderm, when viewed macroscopically, are due to the different thicknesses of the cell layers at the center of the blastoderm. For example, a Stage VII blastoderm is 2-3 cell layers thick at the periphery of the blastoderm and 5-6 cell layers thick in the center of the blastoderm (Figure. 1). If this developmental stage is viewed with the naked eye the blastoderm is a round uniform shape but is a solid white color with no donut shape. When examining a fresh egg, the white embryonic mass of cells may look similar to Figure 2. As development proceeds, cells from the center area of the blastoderm start to die off due to a pre-determined genetic program that tells the cells to die (apoptosis). These dying cells fall into the subblastodermic cavity (Figure 3). The death of the cells progresses producing a transparent thinned-out area in the center of the blastoderm. This thinned area is only one cell layer thick, while at the periphery of the blastoderm the cell mass is still three to five cells thick (Figure 4). This cell shedding produces a Stage X of development and is the most common stage of development in a freshly laid chicken egg. At Stage X, light passes through the thinned one cell layer thick area of the blastoderm, but is reflected off the 2-3 cell layer thick area at the periphery; this effect produces the donut appearance of the blastoderm (Figure 5). The center or transparent region is called the area pellucida and the peripheral area that forms the donut shape is called the area opaca. It is only the cells of the area pellucida that will eventually form the cells of the embryo.

Turkey and duck incubation is 28 days in length compared to the chicken's 21 days. Therefore, it is not surprising that the blastoderm of these two species will appear different from the chicken at the time the hen lays the egg. The blastoderm of a turkey and duck egg is laid at an earlier stage of embryonic developent than the chicken and will most often look similar to a Stage VII chicken blastoderm.

Infertile Germinal Disc Appearance
The germinal disc of an infertile egg usually looks similar to Figure 6. It is quite a small and condensed white mass with an irregularly shaped periphery. Occasionally some vacuoles are present in the center area of the condensed white mass of the infertile germinal disc.

It is important to be as accurate as possible when assessing fertility in freshly laid eggs. It requires some practice and experience, but this skill can be very helpful when one is trying to determine if management changes to improve fertility have been effective.

Figure 1: Drawing of a Stage VII embryo. Blastoderm is 2-3 cell layers thick at the periphery of the blastoderm and 5-6 cell layers thick in the center of the blastoderm.

Figure 2: Picture of a Stage VII embryo. Blastoderm is a round uniform shape, with a solid white color.

Figure 3: Drawing of Stage IX embryo. Cells from the center area of the blastoderm start die off due to a pre-determined genetic program that tells the cells to die (apoptosis) and fall into the subblastodermic cavity

Figure 5: Picture of a Stage X embryo. The center or transparent region is called the area pellucida and the peripheral area that forms the donut shape is called the area opaca.

Figure 6: Picture of a infertile egg. It is a small and condensed white mass with an irregularly shaped periphery.