Reference for Niddah 42:12
כי פליג ר' יהודה אאדומה ושחורה אבל ירוקה ולבנה לא אלא ירוקה ולבנה מאן קתני לה
Our Rabbis taught: If a woman aborted a shapeless object. Symmachus ruled in the name of R. Meir, and R. Simeon b. Menasia likewise gave the same ruling: It must be split, and if there was blood in it the woman is unclean and if there is none in it she is clean. This is in agreement with the Rabbis but also more restrictive than the ruling of the Rabbis. It is 'in agreement with the Rabbis' who ruled that it was possible for the uterus to open without bleeding; but it is 'also more restrictive than the ruling of the Rabbis', since they hold that only where the blood was with it<span class="x" onmousemove="('comment',' Externally, sc. the passing out of the abortion was accompanied by bleeding. ');"><sup>10</sup></span>