Chulin, chapter 9, mishna 6, is discussing the ritual impurity imparted by a dead sheretz (animal of any of certain types). Note that the impurity is imparted only by a dead animal. The mishna says (in my own, loose translation):
A sheretz's egg containing a sufficiently formed fetus does not impart impurity; if one punctured it a bit, it does. A sheretz generated from the ground that is still half earth: if one touches the flesh, he's impure; the earth, he's pure. Rabbi Y'huda says: if he touches even the ground that is near the flesh, he's impure.
(In the latter case, the commentaries explain, Rabbi Y'huda is referring to the earth that would be expected to turn into the rest of the sheretz. In the case of the egg, the puncture gives access to the embryo and thus to the impurity.)
The mishna does not say that these animals (the fetus and the earth-generated sheretz) are dead. Normally, though, a sheretz imparts impurity only if dead. So my question is: Is the mishna referring to a dead animal only, and fails to say so because it's obvious, or is it referring even to a live animal, and a live animal imparts impurity in this case because, not being fully formed, it's not yet considered alive and thus considered dead?