Responsa for Sotah 59:14

ר' אליעזר דתנן ר"א אומר חלה ניטלת מן הטהורה על הטמאה כיצד שתי עיסות אחת טהורה ואחת טמאה נוטל כדי חלה מעיסה שלא הורמה חלתה ונותן פחות מכביצה באמצע כדי ליטול מן המוקף

R. Eliezer-for we have learnt: R. Eliezer Says: <i>Hallah</i><span class="x" onmousemove="('comment',' Part of the dough presented to the priest; v. Num. XV, 17-21. ');">¹⁴</span> may be taken from [dough] which is pure on account of that which is defiled. How is this? There are two portions of dough, one pure and the other defiled. He takes a quantity sufficient for <i>hallah</i><span class="x" onmousemove="('comment',' One twenty-fourth of the whole in the case of an individual and half of that proportion in the case of a baker. ');">¹⁵</span> from the dough from which its <i>hallah</i> had not been removed,<span class="x" onmousemove="('comment',' The pure dough. ');">¹⁶</span> and places a piece less than the size of an egg<span class="x" onmousemove="('comment',' A quantity less than the size of an egg cannot communicate defilement. ');">¹⁷</span> in the centre [of the defiled dough] so that [it may be considered that <i>hallah</i>] had been taken from the mass [of the defiled dough].

Teshuvot Maharam