ZME Controller providing OJT described a loss of separation when the developmental was unaware of a developing conflict and time precluded a second instruction; read back errors were also mentioned as contributory.

Narrative

I was training an R-Side developmental. Just prior to the incident; the trainee climbed Air Carrier X to FL350. Air Carrier X was approximately 10NM northeast of MEI southwest bound; and would be turning west direct JAN when over MEI. Air Carrier Y was level at FL340 southwest bound approximately 20NM north northwest of MEI. Not long before; R43; the ultra-high sector; had been decombined from R45 and R46. I was aware of the route of Air Carrier X; and knew no coordination had taken place with R43; so I asked the trainee if he really wanted to take that action. He was concerned with 2 other aircraft that he thought were a factor. He did not realize Air Carrier X was about to make a significant turn to the W. He immediately corrected the altitude to FL340; thinking Air Carrier X would continue southwest bound parallel to Air Carrier Y. I told him to look at the route. Air Carrier X was in the turn; so I told the trainee to stop the aircraft at FL330; which he did immediately. Air Carrier X's Mode C showed FL323; apparently enough time to level safely at FL330. Not sure of a read back; I asked the trainee; he immediately asked Air Carrier X if he copied. Air Carrier X's Mode C showed FL331. Air Carrier X responded with a rather long answer that he received FL350 and FL340; and that he saw his traffic; and with a comment; the trainee; immediately; reissued FL330. It appeared Air Carrier X was a little slow in responding; however; there was no immediate danger; and he had reported seeing the traffic. Since the error had already occurred; and there was no immediate danger; the trainee and I took no further action; other than to re-establish the proper separation; and advise the FLM. Due to the close proximity of the aircraft; and the pilot not copying the amended altitude of FL330; there was no time for second attempt to avoid the situation. Recommendation; as previously stated; there was no time for a second clearance after catching the pilot's missed read back. I could have possibly taken control; but it is not likely this would have been much benefit due to the close proximity of the two aircraft. Another solution would be to advise the trainee of the potential situation in case he was considering moving an aircraft.