The gross income of an estate or trust is determined in the same manner as that of an individual. Thus, the gross income of an estate or trust consists of all items of gross income received during the taxable year, including:

(a) Income accumulated in trust for the benefit of unborn or unascertained persons or persons with contingent interests;

(b) Income accumulated or held for future distribution under the terms of the will or trust;

(c) Income which is to be distributed currently by the fiduciary to the beneficiaries, and income collected by a guardian of an infant which is to be held or distributed as the court may direct;

(d) Income received by estates of deceased persons during the period of administration or settlement of the estate; and

(e) Income which, in the discretion of the fiduciary, may be either distributed to the beneficiaries or accumulated. The several classes of income enumerated in this section do not exclude others which also may come within the general purposes of section 641.