Cambridge police started to hunt down the unknown assailant of Frank Foster, the 67-year-old Dunster House janitor, yesterday, after receiving from college authorities official notification of the crime.

Consequently, the first important case in many years which may involve undergraduates passed from University jurisdiction to that of the courts. When the assailant is arrested, he will be liable to criminal prosecution.

Condition Serious

Foster, who was taken to the Baker Memorial Hospital Sunday evening after a second collapse from shock, was reported to be in a "serious" condition upon inquiry at the hospital last night.

The possibility that the culprit is a member of the University was given credence in many circles yesterday with a resulting wave of indignation. Since no arrest has been made, however, it is impossible to discard the chance that he may have been an outsider.

Foster was discovered under some bushes in the Dunster House yard during the dance there Saturday. It is likely that an individual, possibly intoxicated, had a quarrel with the janitor while proceeding to the dining room that resulted in physical encounter. Foster weighs only 105 pounds.

Yard Police Arrive

When the Yard police arrived on the scene, they took the names of everyone near the scene of the attack and they are being questioned by the police. All evidence which Colonel Apted's force obtained has been turned over to the authorities.

No statement was forthcoming from University Hall. Apparently, it plans no interference with any prosecution and will only take necessary steps if an undergraduate is involved.