In cell a , what is the structure labeled y ?

microtubules

Explanation:

"Microtubules play a role in the migration of chromosomes to opposite ends of a mitosing cell during the anaphase. Microtubules are hollow tubes 24 to 25 nanometers in diameter composed of 13 parallel rows. The parallel rows are called protofilaments and are made of heterodimers called tubulin subunits."

answer; ///i believe its: ///(embolus);