Scientific American sets forth “an egg-cellent activity from Science Buddies”:
How do animals, such as chickens, which develop inside an egg outside of their mothers’ bodies and therefore do not have umbilical cords, take in oxygen and get rid of carbon dioxide? Bird and reptile eggs have a hard shell. Directly under the shell are two membranes. Between the membranes is a small air cell, also called an air sack, filled with oxygen. As the animal develops it uses the oxygen, which must be replenished, and it also has to release carbon dioxide. How does this happen? Well, if you examine a chicken egg carefully with a magnifying glass, you’ll see that there are tiny little holes, called pores, in the shell. In this activity, we’ll see how those work to let the developing chick breathe.
- Large pot or bowl
- Blue food color
- Liquid dishwasher detergent
- Teaspoon measurers
- Three eggs (for best results, do not use freshly laid eggs, rather, use older, commercial eggs)
- Tongs or large spoon
- Plate or paper towel
- Optional: a sensitive scale, such as a digital kitchen scale or a triple-beam balance that can measure tenths of a gram
- Pour one and one half cups of water in a large pot or bowl.
- Add one quarter teaspoon of liquid dish detergent and one quarter teaspoon of blue food color. Mix well.
- Carefully put the three eggs in the pot with the water, dish detergent and blue food color.
- Make sure that the eggs are submerged in the liquid. If part of the egg is above the surface of the water, mix together liquid dish detergent and blue food color with more water in the same proportions as you did before. Add this to the pot until the eggs are submerged.
- Set a timer for one hour or make a note of the time.
- After the eggs have soaked in the liquid for at least one hour, carefully lift one of them out of the liquid using the tongs or large spoon. How does the egg look?
- Crack the raw egg into a cup, being careful not to damage or crush the shell much.
- Set the empty eggshell on a plate or paper towel.
- Carefully inspect the inside of the shell. What do you see?
- Crack open the other two eggs in the same way. Look all around the inside of their shells, too. What do you see? Do all of the insides of the shells look the same? Are there noticeable differences?
Observations and results
Did all of the eggs have at least a few small blue dots on the inside of their shells? Were the dots mostly clustered in one or a few areas on the inside of each shell?
Directly under the chicken egg’s shell are two membranes. When the eggs are laid by the mother they are warmer than the air, and as they cool the material inside the egg shrinks a little bit. This shrinkage is what pulls the two membranes apart, leaving behind the small air sack that is filled with oxygen. As the developing chick grows it uses the oxygen from the air sack and replaces it with carbon dioxide. The tiny pores in the shell allow the carbon dioxide to escape and fresh air to get in. The chicken egg has more than 7,000 pores in its shell to allow this to happen! These pores also allow water to go through the shell, which is why the dye appears as small dots on the inside of the shell, often clustered in certain areas, and why an egg after being hard-boiled would weigh slightly more than when it was raw. Also, freshly laid eggs do not allow water to penetrate as well as older, commercial eggs do, so fewer blue spots will probably be visible on the inside of fresher eggs compared with older ones.
This experiment was described (somewhat more tersely) some seven centuries ago by Rashba:
בצים שנתבשלו בקדרה עם בשר ואפילו עם קליפתן אסור לאוכלן בכותח, שקליפת הביצה בבירור מנוקבת היא וכשאדם מבשלה בתוך יורה של צבעים תמצא הביצה עצמה צבועה מאותו צבע, ועל כן אסרו בגמרא ביצת אפרוח …1
Taz disagrees with Rashba’s assertion of the porousness of eggs:
וצ”ע ממה שכתב רמ”א בסימן פ”ו סעיף ה’ דאם הביצה נקובה דינה כקלופה דהא כל קליפת הביצה נקובה היא ופולט שפיר ואפילו הכי מותר בביצה אסורה שאינה פולטת אלא (ציר) [זיעה] דמאי שנא נקובה בתולדה או שלא בתולדה …
ואי לאו פה קדוש דרשב”א [הייתי אומר] דאין נקב בביצה בתולדה אלא שבולע ופולט דרך הקליפה מדבר שהוא ממש בו וצ”ע:2
Taz does not explicitly counter Rashba’s argument from the passage of dye through the eggshell. Perhaps he understands that just as טעם is absorbed into the shell and then passes through to the egg’s contents, even in the absence of actual holes in the shell, so, too, is the dye absorbed into the shell itself and then passes through to the egg’s contents.
In any event, Pri Megadim defends Rashba from Taz’s question, explaining that the halachah distinguishes between the natural pores of an eggshell and artificial holes made therein, since the former “are very small” relative to the latter:
ויש חילוק בין נקב בתולדה דהוא קטן מאד … מה שאין כן נקב שנתהוה אחר כך …3