On today’s daf (=page) the Gemara distinguishes between two situations where a pregnant woman dies. If she is suffering from a terminal illness, we can assume that the embryo dies first; if she is killed suddenly, we assume that the embryo dies only after the mother’s passing.
There is a physiological explanation for this statement. A developing embryo relies on its mother for nourishment, oxygen and so forth, and its status depends on the proper functioning of her body. If the mother is suffering from a terminal illness it is likely that the embryo will begin to suffer from faulty circulatory blood flow well before the mother’s death, leading to the embryo’s death. If she is killed suddenly, however, the mother’s death from loss of blood will take place before the embryo dies.
In response to this statement, the Gemara counters that in an actual case the embryo was seen to have a number of convulsions after the mother had already died. The Gemara explains that this is like the tail of a salamander that continues to twitch even after it is removed from the body of the salamander, i.e. that such involuntary convulsions are not a sign of life.
After the limb of a living creature is separated from the body, the nerves of that limb continue to operate in an uncontrolled manner for a short time due to the continued functioning of the neurotransmitters that still send out signals to the limb. Although the muscles continue to flex in response to these neurological signals, this is not necessarily an indication of life.
In contemporary discussions about establishing a working definition of “time of death” and the possibility that “brain death” – a cessation of all recorded brain-stem activity – may be viewed as halakhic death, the Mishnah in Masechet Oholot that discusses the ramifications of cutting off one’s head serves as a key source from the Talmud, opening the possibility of harvesting organs for transplant purposes.