This perek (=chapter) has focused on the rules and regulations that apply to an animal that was slaughtered and was discovered to have a developing fetus within it. The Mishnah on today’s daf (=page) discusses the case of a shilya – a placenta – that was found in such an animal. A placenta is a large organ – filled with blood vessels – that connects the developing fetus to the uterine wall. By means of the placenta – and specifically the umbilical cord connected to it – the fetus “eats,” that is, ingests nutrients that allow it to develop. The presence of a placenta in an animal (or in a human) is a clear indication that a pregnancy began, although if no fetus is present then the pregnancy did not continue with its normal development.
While the focus of the Mishnah is on the permissibility of the placenta – the Mishnah rules that it can be eaten, even though it is not ordinarily considered to be food – it concludes that “it may not be buried at cross-roads or hung on a tree, for these are the practices of the Amorites.”
“The practices of the Amorites” refers to activities that are not truly in the category of idol worship, but they are non-Jewish traditions that are forbidden based on the passage in Sefer Shemot (23:24, and see Rashi there) “…nor do after their doings.” Another source for this prohibition is Vayikra (18:3) “…neither shall you walk in their statutes.” In particular, these are understood as referring to practices of magic and witchcraft.
In explanation of this idea, we find in the Gemara that Abayye and Rava agree that whatever is done for medicinal purposes is not prohibited as Amorite practices, and whatever is not done for medicinal purposes is prohibited as Amorite practices. Thus, the Gemara explains, that the common practice of painting a tree whose fruits fall off with red paint and piling stones on it can be done since the stones may limit the strength of the tree, and by calling attention to it, those who pass by will be encouraged to pray on its behalf.