Whether the yolk of the egg is generated from menstrual blood or from some other moisture.
It is asked further concerning the yolk [vitellum] of the egg, whether it is generated from menstrual blood or from some other moisture.
1. It seems that it is not from blood, because menstrual blood is found in the human nature alone; therefore, the yolk of the egg is not generated from menstrual blood.
2. Moreover, the seed and the nutriment come from the same thing, because seed [semen] is a superfluity of nutriment. But the yolk is the nutriment for the fetus. Therefore, it comes forth from the same root source as the seed. But the seed is not from menstrual blood, and therefore, etc.
To the contrary, the thing generated attests to its principle. But the yolk is yellow [citrinum] or reddish, and therefore it is generated from a reddish moisture. But the blood is such as this, and therefore, etc.
One should reply that the yolk in egg-layers is analogous to the milk in those who lactate, since, just as the milk is the fetus's nutriment, so too the yolk is the nutriment for the fetus, that is for the chick, when it is in or enclosed in the egg, because the chick is generated from the white and the yolk serves as its nutriment. Nevertheless, milk is white and the yolk is red, because milk receives its whiteness from the breasts containing it, which are white and glandular. But the yolk is enclosed in the middle of the white, and it cannot assume a color external to it but may only retain the color nearest to the principle from which the yolk is generated, and the yolk is generated from menstrual blood, as is milk, and for that reason it retains the color of blood.
1. On to the arguments. To the first, one should reply that there are many parts of menstrual blood. One part is converted into the spermatic members, and another part is converted into the members filling up the empty spaces in the bodyfor example, converted into fat [zirbum] and fleshand still another part is converted into the nutriment for the fetus, and another part is superfluous, and that, capable of providing nourishment, is retained in the uterus while the fetus is in the uterus; after its birth, it is cast out. The menstrual blood is not present in flyers with respect to this part, but it is with respect to these other parts. Therefore, since the yolk is the nutriment for the fetus, there is menstrual blood in flyers, from which the yolk is generated.
2. To the second argument one should say that semen and the nutriment of the one generating do proceed from the same root source, because semen is a superfluity of nourishment [ alimentum]. Nevertheless, the seed and the nutriment [nutrimentum] for the one generated do not come forth from the same root source, except perhaps indirectly, and the yolk is the nutriment for the one generated and not of the one generating, and this is why it is not necessary that it come forth from the same root source as the seed does.
Whether the white of the egg is warmer than the yolk, or contrariwise.
It is asked further whether the complexion of the white or that of the yolk is warmer.
1. It seems that the white has a warmer complexion. Because life depends on heat and moisture, then that which is nearer to the origin of life is warmer, and this is the white in comparison to the yolk, and therefore, etc.
2. Moreover, according to Aristotle's fourth book On Meteorology, digestion proceeds by a thickening process. That which is more digested is warmer, and the white is of this sort; therefore, etc.
On the contrary, every astringent is colder. But the white is an astringent, for it is introduced to wounds in order to restrain the flow of blood, whereas the yolk dilates [the blood vessels]. Therefore, the yolk is warmer.
One should respond that among all the humors blood is warmer, with a tempered heat, and therefore whatever is nearer to blood is warmer. But such is the yolk, and therefore, etc.
Moreover, the liver is warmer than the breasts, or blood is warmer than the milk. But the yolk, in comparison with the white, is analogous to blood in the liver. It is therefore warmer.
Nevertheless, a twofold heat can be distinguished. One heat is accidental on account of the member that retains it, and this heat is greater in the white than in the yolk just as the blood that is in the heart is warmer than the blood that is in the liver because of the organ where each is contained. Another heat is a natural heat, and this one is greater in the yolk than in the white.
1. On to the arguments. To the first, one should say that the principle of life is double: near and remote. The principle of life that is nearer is warmer in proportion to its proximity. But both the white and the yolk are a principle of life, since from the white a life comes forth as the result of a power that is in the white, and the yolk is converted into the substance of the one generated, and the yolk is better disposed for conversion into the substance of the one generated than the white is disposed for conversion into the one generated.
2. To the second argument one should say that what is more digested is warmer in comparison to the same thing when it is digesting. This is not the case for the yolk and the white, and therefore the solution is clear.