Whether one who has a defect in some part necessarily generates an animal that is defective in the same part.
One inquires further whether sperm that has been separated off from a certain part may exist in potency to certain other parts. And this is to inquire whether one who has a defect in some part necessarily generates an animal that is defective in the same part.
1. And it seems so. Because nothing is created from nothing. But in one who has been maimed, the matter of the part that has been taken away is as nothing. In such a mutilated one, therefore, there is nothing from which the missing part can be generated.
2. Moreover, everything that is created is created from something proportional to it. But there is nothing proportional to the missing part in the semen separated off from one who has been mutilated. Therefore, this part cannot be created from his sperm.
To the contrary. Frequently one who has been mutilated generates one who is not mutilated, and a cripple generates a non-cripple. But this would not be the case were parts of the sperm determined for determinate parts of the animal.
To this, one must reply that the determinate parts of the sperm are not determined for determinate parts of the animal; rather, it sometimes happens that an animal comes from both homogenous and heterogeneous ones. Now because a plant is uniform, almost any of its parts can become a plant, generating it either by grafting or by putting it in the ground, as is apparent in the willow. Nevertheless, plants that are more noble and complete are generated only from seed [semen], just like complete animals. But sometimes vile things are generated through putrefaction. Therefore, the generative power in an animal can produce an animal from sperm, which is uniform in its parts, so much so that a whole can produce a whole from either the whole or from the part; otherwise, twins would not be generated. Therefore, although the sperm is not separated off from a part that has been removed, nevertheless the power that is in the generative members can dispose the semen toward a homogenous part to which it wants to move, etc. Therefore, the fact that one that is mutilated is generated from one who is mutilated is owing to imperfection of this power, because if the power were potent it could create one not mutilated from one mutilated, and a non-cripple from a cripple.
1. On to the arguments. To the first, one must reply that although the matter of a part that has been removed is like nothing, because that part is not in the semen, nevertheless the sperm that arises from other parts can be disposed to that part and can be made proportional to a uniform part.
2. And in this way a solution is evident to the second argument.
Whether sperm is necessary for the generation of animals.
One inquires further whether sperm is necessary for the generation of animals.
1. And it seems so. Every bodily operation occurs through a bodily organ, as the Philosopher says in that book. But the generation and formation of members is a bodily operation. Therefore, it occurs with a bodily organ mediating it. But such a thing is nothing other than the sperm. Therefore, etc.
2. In addition, two things are required for natural transmutation: an agent and matter. But according to the Philosopher sperm serves as the agent, and the menstrual blood serves as matter. Therefore, of necessity sperm is required.
To the contrary. Sperm and milk arise from the same thing and are ordered to the same end, except that milk is for nutrition and sperm is for completion or composition or maintenance. But milk does not exist in animals lacking breasts. Therefore, neither will sperm exist in animals lacking the generative members. But there are many animals like this. Therefore, etc.
In addition, the Philosopher says that sperm exists in some animals and in others it does not. Therefore, etc.
One must reply that some animals are generated from putrefaction and some are generated by propagation. Sperm is not necessary for generation from putrefaction. It is necessary, however, for the generation of perfect animals. The Philosopher proves this clearly through many indications. For wind eggs do not yield chicks because they are not generated from the sperm's power. Similarly, fish eggs do not produce animals until the male has spread his semen over them. Therefore, one can say that just as a heart or something analogous to a heart exists in every animal, so too in every animal that is not generated from putrefaction there exists sperm or something resembling sperm, as in the ringed animals, as the Philosopher says.
1. To the first argument one must reply that there is a difference between sperm and milk. For milk is only ordered for the nourishment of animals that are generated in the womb, and because there are many animals that are generated outside the womblike those generated by laying eggsnot all animals have milk or breasts, which are the vessels for milk. But sperm is ordered for the generation and formation of the members of every animal, whether it is generated in the womb or outside of it, and this is why sperm is more necessary than milk.
Nevertheless, not all animals have testicles, because testicles exist for the sake of well-being and not simply for being, as is said in the text, since nature has arranged the testicles so that sperm can be conserved and retained until a designated time and so that it not be emitted suddenly. Therefore, animals that lack them have coitus quickly and cannot retain their semen. This is just as it is for animals that have short and straight intestines when compared to animals that have intestines that are twisted and not straight.
2. To the second argument one must reply that although the Philosopher may say that sperm does not exist in some animals, nevertheless he concedes that there is something analogous to sperm in them.
-  "Animals": perhaps, "live young," as is common in the DA.