In the context of a discussion of how the status of a one-day-old baby can affect inheritance laws, the Gemara states that the discussion only has significance if the baby had already been born. According to Jewish law, an unborn fetus is not able to inherit, nor can others inherit him. The Gemara explains that the fetus must have died before its mother has died, so that it cannot be considered a viable person.
A simple explanation for the Gemara’s statement is that 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, or even sudden death, it is likely that the embryo will begin to suffer from faulty circulatory blood flow, leading to the embryo’s demise.
In response to this statement, the Gemara counters that in an actual case the fetus 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 lizard that continues to twitch even after it is removed from the body of the lizard, 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 Mishna in Massekhet 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*.
*For insights into the halakha of organ donation, visit hods.org or download their 2010 paper written in conjunction with the Vaad Halacha of the Rabbinical Council of America.