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Whether the animal is nourished from the yolk.
One asks further whether the animal is nourished from the yolk.
1. It seems not, because "we are nourished by and come from the same things," according to Aristotle in book two of On Generation. But an animal does not come to be from the yolk, and therefore it is not nourished by it.
2. Moreover, the relationship of the white and the yolk is like that of the flesh and the white. But flesh is more suitable as nourishment than the white, and therefore the white is more suitable than the yolk.
3. Moreover, just as the fetus is nourished in the womb of those generating live young, so too are young nourished in the egg of the ones that lay eggs. But in those generating live young the nutriment is not located in the middle of the womb, and in egg-layers, therefore, neither is it found in the middle of the egg.
4. Moreover, every thing that is nourished has an instrument through which it receives nourishment. But the chick in the egg does not have an instrument through which it might receive nourishment, because this only occurs by means of the mouth. Now, however, in the egg the animal's entire head is covered, and the mouth is closed. Therefore, it is not nourished.
The Philosopher asserts the opposite.
One must respond that the yolk is the nutriment for the chick in the egg. According to what the Philosopher says in book nineteen of that book, motion and order depend on the end. But order is double: general and special. According to a general order all animals are ordered to the human, but according to a special order every animal whatsoever is ordered to the continuation of its own species. But in themselves animals are ordered to the human, but they are ordered to the conservation of their own species through their superfluity. For an ox has a power, through its own superfluity, to generate another like itself, but the ox itself passes over to become human nourishment. It is the opposite for plants, because they conserve the species using a part of themselves, but by means of their superfluity, their fruit for example, they are ordered to the human and its nourishment. In the same manner there are two [orders] in the egg. One is ordered to the conservation of the species, and this is the white, out of which the animal is generated, and the other is the one that serves as the nutriment for the animal that is already generated, and this is the yolk. An indication of this is that the more the chick grows in the shell, the more the yellow is diminished, so much so that the whole of it is consumed just as if in a crack in the shell.
1. On to the arguments. To the first, one must say that "we come from and are nourished by the same things," etc. The Philosopher means by this that just as we do not come from just one element, or even from just two, but rather come from them all, thus neither are we nourished by simple elements (for example, from pure air or pure earth, etc.), but rather from a mixture of all the elements, so much so that, just as the four elements come together for our fashioning, so too the four come together for our nutrition or nourishment. Therefore, the Philosopher does not mean that "we come from and are nourished by things the same in number," but from things the same in their proportions.
2. To the second, one must respond that each thing is ordered to a proper end, and because flesh is directed to nutriment and the white is not, flesh is therefore a more suitable nutriment. And in the same manner, because the yolk is ordered to serve as nourishment for the chick and the white is not, the yolk is therefore more suitable as nutriment than the white, since the latter is not ordered to serve as nourishment.
3. To the third, one must reply that the fetus in the womb and the chick in the egg or the egg's shell are quite different things. For the fetus is nourished from without, and the nutriment for the mother and for the fetus are one and the same. And this is why it is unnecessary for the fetus's nutriment to be located in the middle of the womb, whereas the chick in the egg cannot receive its nutriment from the outside because it is enclosed in a shell. And for this reason nature has ordered that the yolk be in the middle, so that nutriment can be sent to all the parts equally.
4. To the fourth argument one should say that just as the fetus in the womb is nourished not by means of its mouth but through the umbilical cord, so too even a chick is nourished in the egg through an umbilical cord. The umbilical cord in flyers, however, is not visible because of its small size. Nevertheless, it does have an umbilical cord near the anus. This is why the Philosopher says that two pathways go forth from the umbilicus, one to the yolk, through which it receives nutriment, and the other to the web, through which respiration occurs.
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