Theses medicae de divina et admiranda hominis in utero formatione

there is not such great dry heat in the uterus that could harden the exterior parts of the seed: furthermore, what need is there for them, when the fetus is contained by the uterus, and neither sweat nor urine is yet collected? Indeed, Galen himself admits that nature applies itself first to the necessary parts.

XXXIIII.
The number of membranes is also rightly doubted. For the middle one, allantoeidēs sausage-shaped membrane, which is called the "sausage-like" membrane in Latin because of the shape of the intestine it resembles, is not a membrane, nor does it consist of membranous substance, but is a certain mucus pertaining to the placenta (as they call it): though in beasts, it is said to resemble the appearance of a membrane.

XXXV.
Therefore, there will be only two membranes, the amnion and the chorion, which arise from the internal membranes of the fetus’s body, namely the peritoneum and the fleshy membrane: it is credible that these, together with other membranes that envelop the principal members, arise in the first delineation.

XXXVI.
The origin and production of these membranes, which our people call the "afterbirth" or "secundines," is miraculous. For the umbilical vessels, in order to reach the liver—the "hepatic" liver, as they call it—more safely, were endowed by nature with two tunics by which they are enveloped. The inner one of these arises from the peritoneum, and the outer from the fleshy membrane. These accompany the vessels until their distribution, but afterwards they expand into an amazing size, and recur, covering the entire fetus like a bladder. In this recurrence they change their turn, and the exterior one, which was continuous with the fleshy membrane, becomes the interior and immediately surrounds the fetus, and is called the amnion. The other, which was interior and arose from the peritoneum, becomes the exterior and surrounds the whole as the chorion, and is called the chorion.

XXXVII.
Others wish not for the membranes, but for the umbilical vessels to be formed first, and emitted like rootlets to draw nourishment into the liver of the uterus: just as we see certain rootlets fashioned in the seeds of plants, by which, once fixed in the earth, the alimentary humor or blood is drawn by certain veins.

XXXVIII.
These vessels serve not for the formation, but only for the nutrition of the fetus: therefore it does not seem probable that they are formed first, since, when the principal members are not yet constituted, they are not needed at all. For the seed as seed is not nourished, but the seminal parts, which can convert blood into seed, are.