On certain properties of eggs.

With respect to this, one asks:

1. Why the yolk sinks when placed in water but the white floats.

2. Moreover, why the yolk is not suitable nutriment until it is cooked although milk and fruit, to which the yolk is analogous, are suitable nutriment without cooking.

3. Moreover, why is it that eggs among flyers are larger in size but fewer in number than are those of fish?

4. Moreover, why do the eggs of flyers have a hard shell, and the eggs of fish have a soft shell?

5. Moreover, why do the eggs of flyers have various colors whereas in fish they are but one color?

[6.] Moreover, why do eggs in flyers have an oblong shape whereas in fish they are round?[1]

[7.] Moreover, why is it that males are more often generated from eggs with an oblong shape, whereas females are more often generated from those that are round?

[8.] Moreover, why do some eggs placed in fire make noise, and some do not?

To the first, one must reply that egg white is very viscous and adheres to that to which it is joined; but the yolk is more permeable and, in addition to this, is denser. Thus, a yolk placed in water is immediately penetrated by the water on account of its permeability, and it sinks on account of its density. But the white floats owing to its viscosity, just as oil does, and because it is strongly adherent, it has the power of clarifying that to which it is added. For it draws all the gross and unclean parts upward and firmly adheres to them, and this is why it clarifies the residue. If, however, vinegar is introduced first, this does not happen because the vinegar is very good at breaking things up. That is why when it is applied to something it breaks it up so effectively into very small pieces, so that the thicker and earthy parts cannot be separated out by the white.

To the second, one should reply that in order for something to nourish, it must be digested. Now, however, fruit is digested by a double heat: both one that is interior, which is natural, and another that is exterior, which is accidental. And this is why fruit can nourish without cooking, but the yolk is only digested by interior heat. Therefore, for it to nourish fittingly, it has to be digested further by fire. And besides this, moisture is repugnant to digestion, and a great deal of moisture abounds in the yolk, since it is generated from the menstrual blood. But earthiness is more abundant in fruits, which is why they especially have a pontic taste owing to their dryness. This is why fruit can be more easily digested than yolk, because the heat of the sun or of the air that contains the heat is adequate to digest fruit, but to digest a yolk a much more piercing heat is needed, like the heat of fire.

Moreover, milk is digested in the breasts, and for that reason it can nourish without further digestion. But the yolk is not digested in something else in this way; this is why, etc.

To the third argument one should reply that fish disperse their eggs in a liquid, and this is why they do not require hard shells. But flyers disperse their eggs in something hard like the earth and this is why they require hard shells, to block contraries and harmful things until the fetus achieves complete formation. Another cause will be revealed.

To the fourth, one should say that variation in color is caused by variation in the first qualities. Now, however, fish have a weak heat, and this is why they do not have a power sufficient to vary the color. But in the flyers there is a stronger heat, powerful enough to separate out things of a different nature, and it can thus vary the colors.

Or, one may say in another way that three things must come together in flyers for the generation of the egg: namely, the male's seed; the white, which is the female's menses; and third, the yellow, which is the nutriment for the fetus. And this is why different colors occur in the eggs of flyers. Nevertheless, because moisture impedes heat and dryness sharpens it, whenever this occurs, the eggshell will be of a different color. For the shell of some birds is white, as in the hen, and in some it tends to black, and in some it is spotted, which occurs when more and then less heat causes separation and purification. But an interior heat is not as strong, and this is why those [eggs] that are interior appear the same in all instances. But it is different in fish, since the male's seed is not necessary for the generation of the egg and the females have eggs from birth [aprincipio], and after they disperse them the male follows and spreads his semen over them, and this is perhaps the reason why fish eggs do not have hard shells. And this is why fish eggs do not have different colors.

To the fifth,[2] one should reply that a thing's magnitude depends on the strength of the agent, but its multitude depends on an abundance of matter. But the egg's matter is more abundant in fish, since they are moist, but the active power is stronger in flyers since they are warm and dry. This is why fish have many eggs, but small ones. They are many on account of the abundance of the matter, but small owing to the weakness of the active power. In flyers, contrariwise, they are large but few in number.

To the sixth argument one must reply that heat moves from the center to the circumference and moves best in a pyramidal shape. This is evident in a fire's flame which, even though it may be broad and wide at the base, tends to form a cone. And for this reason, because heat is stronger in a flyer than in a fish, a flyer's eggs are oblong and a fish's eggs are round, since a weak heat spreads them out into a circle [gyrum].

To the seventh, one should answer that males are naturally warmer than females. A male could be produced from the same seed from which a female is produced if the heat were powerful enough. Therefore, because eggs are oblong due to the strength of the heat and are round owing to its weakness, and because in males there is stronger heat than in females, then males are generated from oblong eggs and females from round eggs. And one must understand that since the pointed end of the egg is warmer and more subtle and the other end is colder and thicker, then the larger part is placed below in the womb and emerges first at the time of laying.

To the eighth, one must reply a great windiness is present in some eggs, and when the shell is violently split in eggs like these, the windiness exits with a great force or noise. And this occurs when the exterior heat is strong. Thus, if a flyer's egg is placed in the middle of a fire, the shell quickly and violently splits, and the windiness exits with a loud noise. But this would not happen if the fire were smaller or the shell had already been split. But great windiness is enclosed in fish eggs, and this is why they make a great noise if they are placed in or spread over fire, as is clear from the herring [allec].

  • [1] The numerals 6, 7, and 8 do not appear in the text but have been added by the translators to facilitate understanding.
  • [2] This is actually a response to point 3 (above).
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