Kommentar zu Wajikra 11:31

הטמאים. the impure ones. How does the Rabbi who did not derive the impurity for the skins of these animals from the word הטמאים explain this word? According to Torat Kohanim on this verse that Rabbi derives from this word that the eggs of such שרצים are impure if they have developed to a stage where they clearly contained an embryo. Me-ilah 17 also teaches that the blood of one category of שרץ and another category of שרץ combine to form the minimal quantity which confers impurity on those contacting same.

כל הנוגע בהם, anyone touching any of the aforementioned creatures (from verse 29 on) when dead, will confer ritual impurity if the size of the creature or its part touched, is at least the size of a lentil. (Sifra)

We have to know what the word אלה is to exclude according to the exegetical approach adopted by that Rabbi. Perhaps we must understand that Rabbi in terms of what Maimonides wrote in the fourth chapter of his treatise Avot Hatumah, namely that the blood of a שרץ conforms to the same rules as the שרץ itself as long as it is part of the animal. The word אלה teaches that when the blood of such a שרץ is no longer part of it, it does not combine with the animal it was taken from to form the minimal quantity which would confer impurity on someone who comes in contact with it.