>>From Point by Point Summary of the Daf:
(m) Version #1 (R. Elazar): This applies only if there is not also a fetus. If there is also a fetus, we are not concerned that (there was a twin, and) the sac contains a (dissolved) fetus.<<
I think this must be amended to - If there is also a fetus, we are not concerned that (there was a twin, and) the sac containED a (dissolved) fetus.
when you write contains that suggests the sac is still closed - but in that case there MUST a foetus inside it because Ein Shilya BeLo VeLAd
Besides if the Silya is closed then the Ubbar found alongside it in the Rechem MUST be a twin - it could not possibly have come from that Shilya which is still closed
meir, Melb Australia
I do not see any implication that the sac is still closed. We can say that it is torn or punctured, and there is fluid inside, and the question is whether we are concerned that the fluid is a dissolved fetus.
I think that the Ra'avad (Hilchos Ma'achalos Asuros 5:13) must learn like I explained, for he explicitly says that a Shilya is Pirsha b'Alma. The only reason to forbid is due to "Chashash Vlad Nimu'ach",
i.e. now there is a dissolved fetus inside. (He argues with the Rambam, who forbids even if the fetus left the sac.)
Indeed, according to the Rambam, there is concern both lest there is a fetus inside, and lest there was a fetus inside. However, it is a Chidush to forbid in the latter case, and I see no indication that Rashi or Tosfos agrees, so I prefer to explain like I did that the concern is for a fetus inside, which all agree to.
Pesach Feldman