The Age of the Earth and the Formation of the Universe
Honors Seminar (UHON 390), Fall 2005

Professor Timothy H. Heaton

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

 

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).