Niddah, Chapter 9, Mishnah 1. Our Mishnah continues the subject of blood that turns up in a place where it need not have come from the woman's body at all. Here the question is about a bedikah cloth, an eid, that was used for an internal examination and then set aside, and about the shape of the stain later found on it.
Here is the situation the Mishnah sets before us. An eid, the cloth with which the examination was made, had been left "nasun tachas hakar", resting underneath the pillow, and afterward "nimtza alav dam", blood was discovered upon it. Picture a woman who examined herself with this cloth, whether before she was with her husband or once they had been together, and then tucked it away beneath her pillow. When the cloth is looked at some time later, there is blood on it.
The ruling turns entirely on the form of the mark. "Agol, tahor" - if the stain is round, she remains tahor. "Mashuch, tamei" - if the stain is drawn out, elongated or oval in shape, she is tamei.
The reasoning follows the principle already established regarding stains: a round mark can be attributed to a louse. A small insect may well have been under the pillow, and when it was crushed its blood left the mark on the cloth. On that assumption, nothing came out of her body at all; the blood came from the crushed louse. That attribution, however, holds only for a mark that is round, since that is the shape a crushed insect leaves. Once the stain is elongated or oval, it can no longer be blamed on the louse, and she is tamei.
The Mishnah signs this halachah with an attribution: "Divrei Rebbi Eliezer", the teaching belongs to Rebbi Eliezer "bar Rebbi Tzadok", that is, the son of Rebbi Tzadok.