Reference for Chullin 9:1
הָעוֹר, וְהָרֹטֶב, וְהַקִּפָּה, וְהָאֱלָל, וְהָעֲצָמוֹת, וְהַגִּידִין, וְהַקַּרְנַיִם, וְהַטְּלָפַיִם, מִצְטָרְפִין לְטַמֵּא טֻמְאַת אֳכָלִים, אֲבָל לֹא טֻמְאַת נְבֵלוֹת. כַּיּוֹצֵא בוֹ, הַשּׁוֹחֵט בְּהֵמָה טְמֵאָה לְנָכְרִי וּמְפַרְכֶּסֶת, מְטַמְּאָה טֻמְאַת אֳכָלִין, אֲבָל לֹא טֻמְאַת נְבֵלוֹת, עַד שֶׁתָּמוּת אוֹ עַד שֶׁיַּתִּיז אֶת רֹאשָׁהּ. רִבָּה לְטַמֵּא טֻמְאַת אֳכָלִין מִמַּה שֶׁרִבָּה לְטַמֵּא טֻמְאַת נְבֵלוֹת. רַבִּי יְהוּדָה אוֹמֵר, הָאֱלָל הַמְכֻנָּס, אִם יֶשׁ בּוֹ כַזַּיִת בְּמָקוֹם אֶחָד, חַיָּב עָלָיו:
The skin [of a slaughtered animal], the broth, the meat dissolved by boiling, that which adheres to the bottom of a saucepan, the fragments of meat adhering to the skin when it is removed from the animal, bones [containing marrow], sinews, horns and hoofs, are computed together to form [with the edible matter or flesh in them] the quantity of the size of an egg, when they are liable to contract and communicate pollution to other edibles, but not the pollution of Nebelah. Thus also, if a person slaughters an unclean animal for a heathen, it pollutes edibles while it struggles, but it does not communicate the pollution of a dead body till life is extinct, or, if its head had been quite chopped off. There are consequently more cases in which edibles contract pollution than there are in respect to pollution by Nebelah. R. Jehudah saith, in reference to the fragments of meat adhering to the skin, "If any of these, when computed together, are of the size of an olive in any one place, guilt is thereby incurred."