One of the few references to the egg in the Bible, and the only injunction connected with it, is the command to drive away the dam before taking the eggs from the nest (Deut. 22:6). The only other references to birds' eggs are in Isaiah 10:14 and the hatching of the egg of the ostrich through the heat of the sun (Job 39:14). Viper's eggs are mentioned in Isaiah 59:5. In contrast, the egg figures prominently in rabbinical literature, both in halakhah and aggadah.
The egg belongs to two spheres of halakhah: as permitted food and as a standard measure of volume (the tractate of the Talmud called Beẓah ("egg") deals with the laws of the festivals and is so called merely because of the first word of its first Mishnah, which deals with the question of the permissibility of eating an egg laid on the festival).
(1) Although it is nowhere clearly stated in the Bible that eggs are permitted for food (the Talmud sees a reference to it in Deut. 22:6; see Ḥul. 140a), on the principle that "that which emerges from a clean animal is clean and that from an unclean animal unclean" (Bek. 1:2) it is established that the eggs of clean birds are permitted for food, and those of unclean birds, forbidden (Ḥul. 122a). With the formation of the hard shell of the egg, however, even before the egg has been laid, it is regarded as independent and no longer part of its dam,
(2) The bulk of an egg is one of the most common of all the measures of volume in the halakhah. It constitutes the usual quantity of volume to establish liability, e.g., for ritual uncleanness, for the size of the *etrog, for the amount of bread from which *ḥallah must be separated, and many others. It is also the standard whereby all other measurements are calculated, a log being equal to six eggs, a kab to 24, and a se'ah to 144 (see Er. 83a and *Weights and Measures). It is evident however that these relative measurements do not accord with the normal size of an egg. It is accepted that the "egg" is that of the chicken (Yoma 80a) and recourse has had to be made to the theory that the egg thus referred to is a "desert egg" which was much larger than the present day one, and to be on the safe side the standard adopted in the halakhah for the egg of the Talmud is two present-day eggs (Ḥatam Sofer, OḤ, Tesp. no. 127).
The egg is regarded both as having laxative qualities and of bringing about sexual stimulation. The egg, being "round and having no mouth" (opening), is regarded as a symbol of mourning which "is like a wheel which continually revolves in the world, and one must not open one's mouth in complaint" (BB 16b; YD 378:9 of Gen R. 63:14). It is therefore given to mourners at the meal given to them on the return from the burial (se'udat havra'ah) and is eaten at the meal before undertaking the fast of the Ninth of *Av. On the seder night of Passover there has developed the custom of eating an egg dipped in salt water before beginning the festive meal. There is no authority for this custom in the sources; various explanations have been put forward, and Moses Isserles connects it with its mourning aspect. According to him it is in commemoration of the destruction of the Temple with which the paschal sacrifice was discontinued, and it happens that the first day of Passover always falls on the same day of the week as the Ninth of Av of each year (OḤ 476:2). A roasted egg, in memory of the festival offering (ḥagigah), forms part of the Passover plate at the seder.
Krauss, Tal Arch, 1 (1910), 124–6; Eisenstein, Yisrael, 3 (1951), 37–40; ET, 3 (1951), 131–45.