It used to be fairly easy to tell when Harvard's varsity football team was preparing for Princeton. Coach John Yovicsin would select an elite group of reserves from Henry Lamar's freshman team, teach them very quickly the arts of the mystical single wing system, and run them against his varsity.

And if someone wandered down to practice anytime during the week before the game, he would hear voices bellowing Princeton's distinctive "TIGER" cadence, and see the football being snapped directly from the center to the running back who would be carrying on the play.

That was the single wing, and until this September, when new Tiger coach Jake McCandless scrapped it in favor of the "T" system, it perennially drove Harvard coaches to drink.

The combination was once unbeatable-Princeton's fine material using a system that nearly every other college had abandoned years ago. A coach could not hope to adjust his defense in a week's time, and he was often left with a painful dilemma. Should he drastically revamp his defensive alignment to stop Princeton, and take a chance that his squad might not be able to return to its regular system a week later?

Or should be make only minor changes and hope for the best? Yovicsin tried both alternatives at times, and won 5-of-12 games at Harvard with them. This fall with the single wing defunct, things have changed.

Harvard's practice yesterday afternoon seemed no different than usual, and in a sense, it was discomfiting. Because of injuries. Monday afternoons at Soldiers Field this fall have seemed like a civilian bomb shelter after a heavy raid. Three times this fall, Harvard has come out of the bomb shelter a winning team. Three times it was not so fortunate.

But each time there was a discouraging list of injuries that at the very least hampered Harvard's progress during the following week. Yesterday was no exception.

Crimson captain John Cranver, who played superbly at defensive end last Saturday against Penn, was hobbling around the sidelines on crutches. Defensive tackle Rick Berne was being treated in the medical room at Dillon Field House for a badly bruised right knee. He couldn't practice. And cornerback Rick Frisbie was danwering around in street clothes, waiting for a dislocated elbow to respond to treatment.

All three should start against Princeton this weekend, but their absence makes Yovicsin's task difficult. The Tigers have put together an impressive offense during the last two weeks, and they are unbeaten in Ivy play. Single wing or not, they will be as bothersome an opponent as the Crimson has had to face this fall, and it is imperative that Yovicsin be able to throw a healthy defense against them.