On chicks.

Further one inquires whether the chick is nourished by the yolk.

1. And it seems not. For an animal with blood is nourished by menstrual blood. Therefore, in order for the yolk to nourish the chick, a member is required to convert the yolk into menstrual blood. But this member is the liver, which is absent when the chick's embryo is first nourished.

2. In addition, menstrual blood is to the human as the yolk is to the bird. But menstrual blood does not nourish a human unless it was digested earlier in the liver. Therefore, neither does the yolk. I inquire, then, by what the heart is nourished before the generation of the liver and the generation of the other members.

The Philosopher says the opposite.

Second, one inquires whether the chick is generated with a mediating, external heat.

1. It seems so. Because the egg does not produce a chick without incubation. But incubation is only to warm the egg. Therefore, etc.

2. In addition, the womb bears the same relation to the embryo as that which contains the egg does to the egg or the egg's contents. But the embryo is created only with the womb's mediating heat. Therefore, the chick is created only with a mediating external heat.

To the contrary. An agent and matter are sufficient for generation. But these exist in the egg that is surrounded by an external heat. This is why, etc.

Third one inquires whether birds of prey have few eggs.

1. It seems not. Because a large number of eggs is a sign of strong power. But there is stronger power in the birds of prey than in others. Therefore, etc.

The Philosopher says the opposite.

To the first, one must reply that the fetus is nourished by the yolk. But the fetus has a double status: one is before the completion of the members, and the other is after. Before the members' completion the fetus does not receive nutriment through the stomach or the liver, but through pathways that resemble veins, and at that time it is not necessary for its nutriment to be menstrual blood, since it is possible for the moisture to resemble it. Nevertheless, after the completion of the members it is nourished by menstrual blood, and this is why at that time it is nourished only by that which passes through the stomach and liver. And the first arguments prove this. In fact, at the same time that the heart is generated the menstrual blood is generated in the heart's ventricles, and the heart is nourished on it until the completion of the other members. But before the heart's generation there is no generation of menstrual blood, and the same is true in the egg.

1-2. With this a solution is evident to the arguments.

To the second question one must reply that there is a natural heat in the egg; but the heat is hampered [ligatus] as the senses are in one who is asleep, and it can therefore not perform its proper operation unless it is moved by an external heat. Now, just as a seed cannot generate by itself unless it fall to the earth (because when it falls to the earth its power is aroused by the earth's heat), so it is for the egg, since its internal heat is aroused by an external heat, and this is why it does not produce a chick unless it has been incubated or warmed by another. Therefore, one must reply to this question that the principal instrument is the internal heat, which, led from potency to act, makes all things. The external heat, however, is a secondary instrument. Therefore, each heat is necessary, and one alone does not suffice in itself.

With this a solution is apparent to the arguments.

To the third question one must reply that a large number of eggs arises from an abundance of moisture at the same time that there is a strong heat. But moisture does not abound in birds of prey. This is because they are good flyers and flight consumes moisture, and also because they have curved talons and large beaks and many feathers, and the superfluity of the menses is converted into these, and, as a result, they have few eggs. And it is because they drink very little, and are therefore dry. Birds with small bodies, however, because they need only a modest amount of nourishment, abound with a great deal of moisture, and this is why they lay several eggs, because they have a lot of material for the eggs. Birds that are good flyers, however, and have, in addition, a heavy body and a lot of moisture produce few eggs at one and the same time (because they are good flyers), but they produce many eggs in succession because they have a great deal of moisture. Pigeons are like this, and they lay eggs many times but lay only a few at the same time, because they only produce two at one and the same time.

To the argument one must reply that although birds of prey have a strong power they lack moisture. Therefore, etc.

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