Rashi on Exodus 34:19/רש"י על שמות ל״ד:י״ט

1כל פטר רחם לי EVERY FIRST OFFSPRING OF THE WOMB IS MINE amongst human beings,

2וכל מקנך וגו׳ AND also ALL THY CATTLE WHICH IT (the mother animal) BRINGS FORTH AS MALE BY AN OX OR AN ASS OPENING its womb: it means, that a male animal opens her womb.

3פטר is an expression denoting “opening”; similar is (Proverbs 17:14) “As if one openeth (letteth run) (פוטר) water so is the beginning of strife”. The ת of תזכר expresses the feminine gender and refers to the animal in labour (i. e. the prefix is 3rd fem. sing., not 2nd masc. sing.: the subject to תזכר must be supplied “the mother animal”).