Chicken Embryonic Development as a Natural Clock

By Paul Bechard

The development of the chicken embryo is divided into stages based on morphological events. The rate of growth for chicken embryos is relatively constant; embryos fertilized at the same time will reach key points in development at roughly identical times. Chicken embryos could be used on two different timeframes. The chick will hatch 20 to 21 days after fertilization, which constitutes the first timeframe. Using the stages of early embryonic development are more accurate for telling time. The first 20 stages, though not separated by equal time intervals, span only a few hours and thus allow an accurate determination of embryo age. Subsequent stages occur in half day intervals, which is much less accurate. The table below shows the 46 stages of chick development and the corresponding timeframes. The attached diagram shows the different morphological stages of chick.

Chick Stages and Incubation Periods
From Hamburger, 1951
Stage Period of Incubation   Stage Period of Incubation
1 --   24 4.0 Days
2 6-7 Hours   25 4.5 Days
3 8-9 Hours   26 4.5-5.0 Days
4 18-19 Hours   27 5.0 Days
5 19-22 Hours   28 5.5 Days
6 23-25 Hours   29 6.0 Days
7 23-26 Hours   30 6.5 Days
8 26-29 Hours   31 7.0 Days
9 29-33 Hours   32 7.5 Days
10 33-38 Hours   33 7.5-8.0 Days
11 40-45 Hours   34 8.0 Days
12 45-49 Hours   35 9.0 Days
13 48-52 Hours   36 10 Days
14 50-53 Hours   37 11 Days
15 50-55 Hours   38 12 Days
16 51-56 Hours   39 13 Days
17 52-64 Hours   40 14 Days
18 65-69 Hours   41 15 Days
19 68-72 Hours   42 16 Days
20 70-72 Hours   43 17-18 Days
21 3.5 Days   44 19-20 Days
22 3.5 Days   45 20-21 Days
23 3.5-4.0 Days      

 

chicken development chart

During incubation, the chick can provide a relatively accurate means of telling time, but environmental conditions must first be met. These include an incubation temperature of 38?C and a humid environment. Temperature must be regulated because this factor significantly impacts the rate at which embryos develop. Lowering the temperature by 1°C is enough to delay development by one day. Thus, in order to use the table above as a reference, and to maintain a constant rate of development (a requirement of natural clocks) the chick embryos must be subject to the same environmental conditions as those used to establish the time intervals. A humid environment is needed so that the eggs do not dehydrate, which results in death of the embryo.

Several problems are encountered using embryonic development as a natural clock. The first is that any given embryo can arrest development at any time, prematurely stopping the clock. Second, there may be large differences in the rate of development from chick to chick even though the eggs begin incubation at the same time. Since one requirement of a natural clock is a constant rate, the variation in developmental rate may significantly alter approximations of chick age. Finally, in order to view a chick embryo, the egg must be windowed, and a microscope must be used to distinguish between early stages.

Although there are numerous drawbacks to using chick development as a reliable clock, it may still be useful in certain settings. For example, a researcher may be able to identify which batch of eggs a chick belongs to by comparing the stage of the unknown chick to the stages of the eggs in several different batches. He or she would also be able to estimate how long a batch of eggs has been in an incubator based on the stages of several chick embryos.

Sources:
  1. Hamburger, Victor; Hamilton, Howard L. January, 1951. A series of normal stages in the development of the chick embryo. Journal of Morphology, 88(1).