662 Ulisse Aldrovandi

cannot accommodate the matter; and therefore, the material, conjoined at the same time, forms only a single head for the twins. However, this can also result from the violent movement of the mother; hence we often observe that twins in their upper parts are governed by a single, very large head.

Nor should monstrous parents be excluded from these causes. If both were composed with some superfluous part, a monstrous offspring with numerous parts could easily result from them, as Nature always strives to perpetuate something similar within a species. Although this is not always necessary, for sometimes the portion of seed assigned to the formation of the fetus from the parents' extra parts is too small for those parts to be doubled in the offspring.

When twins, otherwise complete in all their parts, are conjoined in the maternal womb in various ways—as has been shown in many of our illustrations—we believe this originates from several causes. First, a woman carrying twins might experience a violent physical jolt, a fall, or some other cause of this kind, which can cause the tender substance of the fetuses to coalesce. And since twins do not always occupy the same position in the womb, it follows that two-bodied births are sometimes joined along the length of their bodies, and sometimes at the buttocks.

Sometimes fetal diseases can also be the cause of twins being born fused. This happens when some acidic humor flowing to the womb erodes the tender surface of the twins; once the discharge is checked, those damaged parts of the infants fuse into one, and the twins emerge into the vital air joined at the front, back, or side.

What we have asserted so far regarding human twin births must also be confirmed for the causes of two-bodied animal fetuses, since Nature always operates in both cases according to the disposition of the material and the presence of obstacles.

Regarding those two-bodied monsters that have another imperfect body of smaller stature attached, it should be noted that this usually results from superfetation. For although this is rare, it is reported that rare monsters of this kind have also been observed. Moreover, almost all authors acknowledge superfetation, and daily experience and observation confirm it. Cardano first observed superfetation in his day in his commentaries on Hippocrates. Indeed, Dodoens, in his annotations to the third chapter of Benivieni, recounts that the wife of an honorable citizen, in the year of our Lord 1560, on the seventh day of December, gave birth to a healthy child; the following day, the eighth of the same month, she gave birth to another fetus, not yet four months old, which as yet showed no form of eyes, nose, or mouth. This clearly argued for a subsequent conception and, consequently, superfetation. Finally, Alexander Benedictus also records a monstrous superfetation, to whom we refer our readers.

Since superfetation does occur, it can happen at different times during pregnancy. If it occurs in the early stages of gestation, the seed adhering to the other conception can create a new part within it—as was observed in that monstrous fetus which, besides its own head, carried another in its belly, a monster we discussed in the chapter on two-headed monsters.

However, if superfetation happens in the second or third month of pregnancy, the second fetus will be smaller than the first. As it grows, perhaps due to the cramped space or some other cause unknown to us, it becomes attached to the larger fetus, as was shown in those instances where another imperfect little body hung from the belly or chest. Thus, by reason of superfetation and the difference in conception times, one body of a twin birth always ends up larger than the other, since they were generated at different times. By contrast, true twins conceived at the same time—whether separate or fused—always share an equal body mass.

Finally, the causes of two-bodied beings composed of both human and animal natures must be drawn from those universal causes concerning fetuses made of parts from different species. From corrupt seed degenerating into a foreign nature, even within the womb, beasts, frogs, and serpents are produced, as was explained previously; and so we believe an infant was born with a serpent adhering to its back