On birds' eggs.
We have already discussed the cause of sterility," etc. In this seventeenth book one inquires first whether there ought to be a distinction among the parts in birds' eggs.
1. And it seems not, because there is a greater heat in birds' eggs than in fish eggs; but there is greater heat in a woman than a bird. Therefore, since there is no distinction among parts in a woman's seed [semen], a distinction will exist all the less in the seed of birds.
2. In addition, the middle savors the nature of the extremes. But in the first generation of the egg the whole egg is yellow, and in the last generation the whole becomes liquid. Therefore, it will be uniform in the middle just as it is in the extremes.
The opposite is evident to the senses.
Further one inquires whether the parts of the egg are rightly ordered.
1. It seems not. For things below imitate things above, as far as possible. But in this universe the earth is in the middle. Therefore, it is true in others as well that that which is more earthy will seek the middle more. But the shell is earthier than the yolk. Therefore, the shell is in the middle and the other parts will surround it, just like the airy or watery parts.
2. In addition, whiteness is a sign of coldness, as is apparent in snow, and redness a sign of hotness, as is apparent in bile and honey. Since, then, the hot should seek the circumference and the cold should seek the center, it seems that the white of the egg will be inside toward the center, and the yoke will be toward the circumference, since it is red and hot.
The opposite is apparent to the senses.
Third, one inquires whether the larger part of the egg ought to come out first.
1. It seems not. For that which is more pointed penetrates another more easily. Therefore, the pointed part is better suited for coming out than is the larger part.
2. In addition, in the ninth book of this work the Philosopher says that animals come out on their head. But the chick's head is toward the pointed part of the egg. Therefore, the pointed part will precede the other when coming out.
The Philosopher says the opposite.
To the first, one must reply that there are distinct parts in eggs. And the reason for this is that the nature that produces the one generated provides for it whatever things are appropriate for its generation. Now, however, an animal does not exist without nutrition, and this is why, when an animal is generated from eggs outside a uterus, nature provides not only that there is something in the egg from which the animal can be generated, but also provides something by which it can be nourished while it is enclosed within the eggshell. First, there is the egg white; second, there is the yolk. Therefore, the egg white provides something in place of the sperm as matter for the fetus, and the yolk provides its nourishment.
Nevertheless, the reason why these things are distinct in birds' eggs but not in fish eggs can be threefold: one is the incomplete nature of the eggs in fish; another is the heat's weakness, because it cannot separate from each other those things which are distinct; and the third cause is that the fish egg does not result merely in nutriment, or that it is not nutriment simultaneously with the semen, as it is in birds' eggs.
1. On to the arguments. To the first, one must reply that a bird's egg and a woman's menses are quite different, because a human is formed inside the uterus, and this is why it is unnecessary that there be two parts to the menses that is received into the woman's womb, one of which is equivalent to the matter for the fetus and the other for its nutriment. This is because the menstrual blood on which the fetus is nourished is continually replenished as the woman is nourished. But a bird is formed outside a uterus, and this is why it is necessary that each exists in the egg: both something in place of the menses and something in place of the nutriment.
2. To the second argument one must reply that since the heat is weak in the beginning, the parts are not distinct and the whole appears to be yellow. But when the heat is strengthened, it distinguishes the parts, whereas at the end, when nature intends to delineate and shape the matter by the power of both the internal and external heat, the matter liquefies, so that it may receive the intended form more easily. And this is why, etc.
To the second question one must reply that nature acts for the sake of an end. Therefore, it acts from intention, because it is not possible to be absolved from the intention toward an end. Since, then, the chick is formed from an egg outside a uterus, nature arranged a hard shell in order to protect what is inside from corruption. And in addition, heat seeks the circumference, and cold the center: this is why the white, which is hotter, is outside the yolk. And, besides this, the yolk is earthier. An indication of this is that if it is cooked, it appears dry and earthy after it has been cooked and the moisture has evaporated. And this is why the shell is naturally situated on the outside as a protection from things contrary to it, and the white is nearer to it, because it is hotter and more airy, and the yolk is in the middle owing to its earthiness.
1. To the first argument one must reply that in some cases nature operates among things below by imitating things above. In other cases it does not operate by imitating things above, because it operates from an intention to a determined end at which it may not arrive by imitating things above. But if the exterior part of the egg were soft, it would easily be corrupted when the egg is deposited on the ground. And this is why it is hard outside.
2. To the second argument one must reply that whiteness arises from two things. Sometimes it arises from cold and from the heat's weakness, as is apparent in snow and frost and many other things that are undigested. And sometimes it arises from a complete digestion afforded by heat, as in the case of charcoal ashes and in milk. Since, then, the egg white is more completely digested than the yolk, and is therefore whiter than the yolk, it is not from coldness, as the foolish physicians say, but from the completion of digestion, etc.
To the third question one must reply that the larger part of the egg comes out first because it is heavier and the heavier descends more quickly, because the more pointed part is joined to the womb and for this reason it is separated from it more slowly, and also because the more pointed part is hotter. And an indication of this is that females are generated from round eggs and males are generated from oblong ones, because heat always tends to form a pyramidal figure.
1. On to the arguments. To the first, one must reply that the pointed part divides more easily, but the larger part is more inclined to descend.
2. To the second argument one must reply that there is a difference between the exit of animals generated in the belly and the exit of birds, since animals generated in the belly exit onto their heads, but birds exit upon their feet.
With respect to what is said in the ninth book, one must have an understanding of those that generate animals in the belly.
-  Reading secundum for the text's secundo.