(a) Yochanan Kohen Gadol was - Shimon ha'Tzadik's successor as Kohen Gadol (see Tiferes Yisrael).
(b) His negation of Viduy Ma'asros was due to a Takanah of Ezra - who transferred the Mitzvah of giving Ma'aser Rishon to the Levi'im from the Levi'im to the Kohanim (see Tos. Yom-Tov).
(c) And he therefore negated it - since the Yisrael was no longer able to say 've'Gam Nesativ la'Levi'.
(a) He also nullified ...
1. ... the Me'orerim, with reference to the Levi'im - who used to cite the Pasuk in Tehilim "Urah Lamah Sishan", which might be misconstrued to mean that Hash-m is actually sleeping (see Tos-Yom-Tov and Tiferes Yisrael).
2. ... 'the Nokfim', with reference to the Kohanim - who used to make a scratch between the horns of kid-goat Korban, causing it to bleed, so that it would not see what is happening and make it difficult for the Kohanim to tie it up and Shecht it.
(b) He nullified it - because it resembled a Blemish (see also Tiferes Yisrael).
(c) In its stead - he instituted the twenty-four rings on the floor besides the Mizbe'ach, into which the heads of the animals fitted.
(a) Until the time of Yochanan Kohen Gadol, the Tana concludes, the sound of a hammer could still be heard in Yerushalayim and one needed to query the Am ha'Aretz from whom one purchased crops about Demai ('Da Mai'?). When ...
1. ... by implication, the Tana says that Yochanan Kohen Gadol negated the sound of the hammer in Yerushalayim, he means - that he stopped the use of hammers there on Chol ha'Mo'ed, even for a Davar ha'Aved (to prevent a monetary loss, which is generally permitted), because the sound of hammering throughout Yerushalayim had reached such proportions as to be considered a denigration of Chol ha'Mo'ed.
2. ... he says that in his time, one no longer needed to ask the seller about Demai, he means - that Yochanan Kohen Gadol instituted the obligation to separate Terumas Ma'aser and Ma'aser Sheni from all fruit purchased from an Am ha'Aretz.
(b) He instituted this - because not realizing that Terumas Ma'aser was Chayav Misah is subject to Misah bi'Yedei Shamayim no less than Terumah Gedolah (the only Matanah that they all took seriously).
(c) He did not include Ma'aser Rishon and Ma'aser Ani in the Takanah - because since there is no intrinsic Isur in eating them, he applied the principle 'ha'Motzi me'Chavero, Alav ha'Re'ayah' (If the Levi and the poor man claim Ma'aser, let them prove that the original owner had not Ma'asered them), particularly bearing in mind that most Amei ha'Aretz separated Ma'asros anyway.
*** Hadran Alach 'Kerem Revai' ve'Salik Maseches Ma'aser
On to Chalah