The defendant was charged in a complaint in a District Court with delinquency in that on or about October 31, 1965, he "did assault and beat A. Kevin Trainor by means of a dangerous weapon, to wit, a razor." After an adjudication of delinquency, the defendant appealed to the Superior Court (see G. L. c. 119, Section 56) where the case was tried subject to the provisions of G. L. c. 278, Sections 33A-33G. In that court the jury returned a verdict of guilty and the defendant was adjudicated a delinquent and committed to the Youth Service Board. The defendant appealed. Three assignments of error arise from his exceptions to the denial of his motion for a directed verdict on the Commonwealth's opening (assignment No. 1), to the denial of such a motion at the close of the Commonwealth's case (assignment No. 4), and to the denial of such a motion at the close of the case (assignment No. 5). The first assignment need not detain us, for "[a] judge cannot be required to direct a verdict on an opening." Perry v. Carter, 332 Mass. 508 , 509. Mallard v. Waldman, 340 Mass. 288 , 290. Assignments 4 and 5 present substantially the same question and will be considered together. The judge rightly denied the motions for a directed verdict. The evidence need not be recited. The evidence introduced by the Commonwealth clearly warranted a finding that the defendant, without legal justification, assaulted Trainor with a dangerous weapon (a razor) and inflicted serious wounds on his face. The defendant introduced evidence tending to show that he was acting in self-defence. But the jury could disbelieve this evidence; or, if they found that the defendant was acting to protect himself, they could have concluded that he used means that were not reasonably necessary. See Commonwealth v. Peterson, 257 Mass. 473 , 478; Commonwealth v. Kendrick, ante, 203, 211-212. The only other assignment argued (No. 2) reveals no error. The testimony of Officer Tyler that as a consequence of conversations with the victim and his three companions, he "went and looked for . . . [the] defendant" was properly admitted. Commonwealth v. Moulton, 4 Gray 39 . Commonwealth v. Feci, 235 Mass. 562 , 567. Commonwealth v. Agiasottelis, 336 Mass. 12 , 14.