The opening pages of this new tractate, called Beitzah [lit, Egg] deal with the question of whether an egg that is laid on Yom Tov may be eaten. Perhaps it is mukzeh, that is, forbidden to be touched or moved because it was produced on a holiday, and not designated as food the day before. In explaining the opening Mishnah, the Babylonian sage Rabbah (d. 320 C.E.) makes this statement:
ביצה ב, א
כֹּל בֵּיצָה דְּמִתְיַלְדָא הָאִידָּנָא — מֵאֶתְמוֹל גָּמְרָה לָהּ
any egg laid now was already fully developed yesterday, and merely emerged from the chicken today.
Since a lot depends on whether or not this statement is biologically correct, today we are going to discuss how a chicken actually lays her eggs. Ready? Let’s go.
From here.
The Biology of Egg Production
According to The Poultry Site (“Your poultry knowledge hub”) it takes about 25 hours for a chicken to grow and lay her egg. First, the hen produces a small ‘egg’ (with no shell) from her ovary, and - fun fact- the chicken, unlike most animals (including us) has only one ovary, instead of the usual two. Here is a map so that you can follow along:
Reproductive organs of the hen. From here.
At this stage, that egg is one of the yolks of the 4,000 with which the chicken is born. It then moves out of the ova and into the infundibulum. If there is a boy chicken around, this is where his sperm would meet the yolk and fertilize it. (You know that tiny whitish spot you see on every egg yolk? That's a single female cell called a blastodisc. If it is fertilized by the boy chicken’s sperm, cell division and embryo development begins there.) But there are no boy chickens around when the goal is to produce eggs for consumption. So the unfertilized yolk continues on its journey and passes through a section called the magnum, where the albumen, the white bits of the mature egg, are secreted around the yolk. The egg is now about 3 hours old. As it approaches the end of its journey along the hen’s reproductive tract, the inner and outer layers of the shell membranes are added, a process that takes about another hour.
Now the egg is in the hen’s uterus. This is where the shell, made mostly of calcium carbonate, is added. It takes about 20 hours to form and perhaps another hour for the color, or pigment, to be applied to the outer eggshell. This is where the eggshell gets its color, whether blue, white, brown, or green. Finally, the completed egg in its fully formed shell moves through the hen’s vagina and into her cloaca. That takes only a minute, and then, pop, out comes a nice fresh egg.
So was Rabbah correct? Kinda, but not really. It takes about 24 hours from start to finish, but once the egg is “fully formed” it is laid immediately. A fully formed egg does not sit around inside the chicken, which is what Rabbah seems to have been suggesting. On the other hand, the process of forming an egg takes a whole day, so in that sense, Rabbah’s egg laying biology was close to what actually happens. So now you know.
