Indicative.—The secondary endings of the first aorist active were originally added to the stem ending in -ς-; thus, ἐλυ_ς, ἐλυ_ς-ς, ἐλυ_ς-τ, ἐλυ_ς-μεν, ἐλυ_ς-τε, ἐλυ_ς-ντ. From ἐλυ_ς came ἔλυ_σα (by 35 c), the α of which spread to the other forms except in the 3 sing., where ε was borrowed from the perfect.
a. In the middle the secondary endings are added to the stem ending in -σα-. For the loss of ς in -σο, see cross465 b.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].