Sotah 30B

וחכמים אוסרין.

And the Rabbis prohibit separating ḥalla in this manner.

ותניא כביצה.

And it is taught in a baraita that R' Eliezer even allows the ritually pure dough placed in the middle to be as large as an egg-bulk, even though dough of that size is susceptible to the halakhot of ritual impurity.

סברוה אידי ואידי בעיסה ראשונה, וחולין הטבולין לחלה לא כחלה דמו.

The Talmud now explains the reasoning of those who tried to prove from here that R' Eliezer is of the opinion that 2nd-degree ritual impurity cannot impart 3rd-degree ritual impurity upon non-sacred items: They assumed that both this Mishnah and this baraita are referring to cases where the dough is of 1st-degree impurity. And furthermore, they assumed that all the tanna’im agree that non-sacred food that is untithed with regard to the obligation to separate ḥalla, as its ḥalla has not yet been separated, is not treated like ḥalla as far as its ability to contract 3rd-degree ritual impurity. Rather, it is regarded as generic non-sacred food, which is susceptible only to 2nd-degree impurity.

מאי לאו בהא קמיפלגי, דמר סבר: אין שני עושה שלישי בחולין,

Based on these assumptions the Talmud explains how these authorities understood the tannaitic dispute: What, is it not clear that R' Eliezer and the Rabbis disagree with regard to the following matter: One Sage, R' Eliezer, holds that an item of 2nd-degree impurity cannot impart 3rd-degree impurity to non-sacred items. Therefore, there is no problem placing an egg-bulk of pure dough in the middle, as although it will touch the impure dough and will thereby contract 2nd-degree impurity, nevertheless it is unable to transmit impurity to the pure dough.