Legacy to a deceased legatee.) Unless the testator expressly provides otherwise in his will, (a) if a legacy of a present or future interest is to a descendant of the testator who dies before or after the testator, the descendants of the legatee living when the legacy is to take effect in possession or enjoyment, take per stirpes the estate so bequeathed; (b) if a legacy of a present or future interest is to a class and any member of the class dies before or after the testator, the members of the class living when the legacy is to take effect in possession or enjoyment take the share or shares which the deceased member would have taken if he were then living, except that if the deceased member of the class is a descendant of the testator, the descendants of the deceased member then living shall take per stirpes the share or shares which the deceased member would have taken if he were then living; and (c) except as above provided in (a) and (b), if a legacy lapses by reason of the death of the legatee before the testator, the estate so bequeathed shall be included in and pass as part of the residue under the will, and if the legacy is or becomes part of the residue, the estate so bequeathed shall pass to and be taken by the legatees or those remaining, if any, of the residue in proportions and upon estates corresponding to their respective interests in the residue. The provisions of (a) and (b) do not apply to a future interest which is or becomes indefeasibly vested at the testator’s death or at any time thereafter before it takes effect in possession or enjoyment.