Pictograph of a turtle shell or sprouting seed bursting through its husk - oracle-bone forms show a cracked rectangle with a tail dropping below. The same image powers all senses: armour, fingernail, and 'first' as in 甲乙丙丁. Indexed under Kangxi #102 田 by graphic resemblance, though 甲 isn't etymologically a field.
Filed under radical 田 (tián, #102) by convention. 田 is not a separate component in 甲, so no strokes are highlighted.
