הלכה על חולין 226:21

And they, may their memory be blessed, said (Temurah 33a) that the ashes of meat [that was cooked together] with milk is prohibited; like the ashes of all things that are prohibited to benefit from, which require burial. And they also said (Chullin 111b) that the Torah only prohibited meat with milk, regarding the milk of a living beast. But with the milk of a dead [one], it is not prohibited . And therefore the udder is permitted [to be eaten] with its milk, according to Torah writ. However the sages forbade it as a fence, until one removes its milk from it, as is elucidated in its place (Chullin 109b). And there are two dispensations for milk that is found in the stomach [of an animal]: one, that it is included in [the category of] the milk of a dead [one]; and also since it is only like refuse more generally - as it has already been digested there. And therefore it is permissible from the outset. And it is not necessary to say that that which is found coagulated there is permissible - as it is certainly like refuse - but even that which is liquid was permitted by the Geonim. One who cooks an embryo in milk is liable, and so [too,] one who eats it. But one who cooks a placenta or skin or tendons or bones or the roots of the horns or hoof nails is exempted. [These] and the rest of the commandment's details are elucidated in the eighth chapter of Chullin.