The controversial dismissal of England wicketkeeper Matt Prior was a flashpoint moment as Middlesex took complete control of their County Championship Division One clash with top-of-the-table Sussex at Lord's.
At the close of day two Sussex were still 352 runs adrift at 147-7 in reply to Middlesex's first-innings total of 499-8 declared.
The follow-on now beckons and Middlesex, who began this game just 10 points behind Sussex in third place, are sniffing blood against the only other unbeaten team in the division.
One of the Sussex wickets to fall was that of Prior, who was given out for 10 to a low bat-pad catch at silly point by Sam Robson, off spinner Ollie Rayner. Before walking from the field though, Prior seemed to indicate angrily twice to the umpires, Martin Saggers and Alex Wharf, that he did not believe Robson had taken a fair catch.
Pointing with his bat to the ground, Prior looked as if he was claiming that the ball had touched the ground as Robson dived forward to scoop up the ball. Saggers, standing at the bowler's end, consulted with Wharf, who was stood at square cover, before giving Prior out.
Several of Middlesex's close fielders, however, including keeper John Simpson, did not appeal very loudly for the catch, which in itself was strange given the importance of Prior's wicket.
His departure left Sussex on 92-5, and their position became even more perilous by the close as Rayner finished the day with 3-47 from 17 overs.
There were wickets too for the other frontline Middlesex bowlers, with James Harris looking particularly dangerous in a spell from the Nursery End which brought him the scalps of Mike Yardy, lbw for 22 to an inswinger, and Chris Jordan, also leg-before, for nought.
Earlier, the start of Sussex's reply was undermined by Tim Murtagh, who claimed his 31st championship wicket of the season when he had Chris Nash caught at third slip for 13, and Toby Roland-Jones, who struck just before tea to send back Luke Wells for 19, skying an attempted pull to mid wicket.
Rayner's first wicket was that of Sussex captain Ed Joyce, who made the fateful decision to bowl first on the opening day and was now stumped for 20 when he stretched forward to drive his 21st ball, missed and was smartly stumped by Simpson as he overbalanced.
The day began with Middlesex on 320 for three, and Chris Rogers took his overnight 161 to 184 before he attempted to hit Steve Magoffin over the top and was well-held by Yardy back-pedalling from mid off.
Rogers faced 305 balls and hit 22 fours, in 406 minutes at the crease, and his fourth-wicket stand with Neil Dexter brought 155 runs to ensure a significant Middlesex total.
Dexter completed an excellent hundred of his own, before edging Will Beer to slip on 104, from 197 balls and with 11 fours.
Simpson's 50 from 93 balls kept Sussex on the back foot, but it was 39 from just 25 deliveries from Gareth Berg, with six fours, which provided the major acceleration needed before the declaration came in mid-afternoon.