Section 2. Whoever is guilty of murder committed with deliberately premeditated malice aforethought or with extreme atrocity or cruelty, and who had attained the age of eighteen years at the time of the murder, may suffer the punishment of death pursuant to the procedures set forth in sections sixty-eight to seventy-one, inclusive, of chapter two hundred and seventy-nine. Any other person who is guilty of murder in the first degree shall be punished by imprisonment in the state prison for life. Whoever is guilty of murder in the second degree shall be punished by imprisonment in state prison for life. No person shall be eligible for parole under section one hundred and thirty-three A of chapter one hundred and twenty-seven while he is serving a life sentence for murder in the first degree, but if his sentence is commuted therefrom by the governor and council under the provisions of section one hundred and fifty-two of said chapter one hundred and twenty-seven he shall thereafter be subject to the provisions of law governing parole for persons sentenced for lesser offenses.