Whether all members are generated simultaneously.
Next one asks about the formation of the members. And first, whether all members are generated simultaneously.
1. And it seems that they are. For according to the Philosopher in the second book of On the Soul "the acts of active agents are present in the one undergoing action or being disposed for it." Thus if two materials are equally disposed when an agent is present, they will receive act at the same time. But the power in the sperm acts on the whole menses, in which all the members exist in potency. Therefore, it will form one member for the same reason as another, since it acts equally on every part of the menses.
2. In addition, the fact that an agent is acting on something now and did not do so earlier is because it was waiting for something that was lacking. If, then, a power forms one member now and not another, it is necessarily waiting for something that is needed for the formation of the other member. But this is not the case, for the semen of male and female are sufficient to produce an animal.
The Philosopher says the opposite.
To this, one must say that not all members are generated simultaneously. This is manifestly clear to the senses, because the interior members are formed before the exterior ones, and the heart is formed before others. It is also seen by reason. For everything acting by means of natural transmutation needs time for its action, and everything acting by intention produces earlier what it most intends to do. The exception is those that are ordered for the sake of another, because in such as these that which is first in intention is last in execution. Now then, the power in the semen acts both by transmuting and through intention. It is necessary, then, that nature first stabilize something upon which it will establish the thing it has acted upon [suum operatum] (that is, its object). And this is why it first produces the member that is more necessary and lastly the one that is less necessary.
Or in another way one can say that generation intends two things: either the last goal [terminum] of an entire process of change, or the entire change preceding the last goal. If generation occurs in the first way, then not all members are generated simultaneously because the nature of each member is not completed simultaneously. If generation occurs in the second way, it can be said that all members are generated simultaneously because all members are on the path to completion simultaneously.
1. <On to the arguments>. To the first, one must respond that although all members exist in potency in the semen simultaneously, nevertheless the agent first intends that which is more necessary, and this is why it acts more toward its production and produces it sooner.
2. To the second argument one must reply that it does not produce all the members simultaneously because it needs something, since the matter is not best prepared [disposita] for all members. And this is why, before it introduces the form for all the members, it requires that the matter be prepared for all the members, and the agent cannot do this at the same time; rather, first it prepares [the matter] for one and then for another. In this way a solution to this question is apparent.
Whether the heart is generated first.
Further one inquires whether the heart is generated first.
1. It seems not. For a natural power operates by means of similar parts, but the natural power is prior to all other powers, for the fetus is nourished first, etc. Therefore, a homogenous part is generated first, and therefore, not the heart, which is an official member.
2. In addition, the Philosopher argues that the heart is the first member because, once the other members have died, the last operation remains in the heart. But one can make the same argument for the liver, because, once all the other powers have ceased, a power operates last in the liver, as is evident among paralytics.
3. In addition, those that exist for an end precede the end in generation. But the brain exists for the sake of tempering the heart, and the liver for its nutrition (that is, to restore what has been lost). Therefore, the liver and the brain precede the generation of the heart.
4. In addition, a moisture is badly bounded by its own boundary, but well by that of another. In order for moisture to sustain its action, it is necessary that it be bounded elsewhere and be contained in something else. But the semen is contained in the placenta [secundina]. Therefore, the generation of the placenta precedes the generation of every other member.
The Philosopher says the opposite.
To this, one must say that a part is of two types. One is necessary to a thing, resulting in the thing's creation [constitutio], and another is a part that does not result in the thing's creation but is said to be a part because it is necessary for generation and is formed first from the matter of generation. If we are speaking in the first way, the heart is generated first. If in the second way, then it is the placenta, which is a sort of membrane containing the semen until it is digested and until the form of the fetus is introduced and completed. The placenta arises from a certain portion of the semen itself, but the placenta exits last with the fetus and does not result in its creation. Thus it is the first thing generated from the semen, but only the heart is the first part of the animal generated from the semen. And the reason for this is that the first power is the vital power, for nothing is nourished, grows, or has sensation unless it is alive. Thus all the powers have their roots in the vital power. For the liver only operates with a mediating heat, and sensation cannot occur without heat and motion, as is said in books two and three of On the Soul. But the root of heat is in the heart, and this is why the heart is the first part that is generated in an animal, and after the heart comes the liver. Thus Avicenna says that two bubbles appear first in the semen: out of one arises the heart, and out of the other the liver. Nevertheless, the soul is received first into the heart, and this is why the heart is said to be the seat of the soul. The Philosopher demonstrates this many times in the text.
1. On to the arguments. To the first, one must reply that a part is called "official" because it is deputed to some task [of ficium], and thus both the heart and liver are official parts. A part is called "official" in another way, because it is composed of diverse parts, and thus these dissimilar parts are called "official." Therefore, the heart can be called a homogenous part in one way, because it has the same nature as the whole and the part, and it can be called official in another way. When it is said in the first argument that a natural power operates by means of homogenous parts, one can say that in one way the heart is a homogenous part. Or one can respond in another way to this argument that a natural power is not first, but the vital power is prior.
2. To the second argument one must respond that every power has its own corresponding operation. Thus, although motion and sensation, which are operations of the animal power, are lacking in paralytics, they are not absent by virtue of a lack in the natural power, which is in the liver. Still this one is not absolutely first itself, because when the natural power is absent the vital power can remain in the heart.
3. To the third argument one must reply that some things are ordered to other things as if to an end in two ways. They can be ordered either for generation, and ones such as these must precede, or for conservation, and ones such as these can follow. But the liver and the brain exist for the sake of the heart, not for its generation but for its conservation, and this is why it is unnecessary that they be generated first.
4. To the last argument one must respond that although the placenta is generated from the semen, nevertheless it is not part of the animal, etc.
-  The origin and function of the secundina are fully discussed by A. in DA. See for example DA 22.214.171.124 (SZ 1: 638-39).