ct al ▁ex cip ul um ▁( the ▁outer ▁layer ▁of ▁t issue ▁compr ising ▁the ▁ap othe cia ) ▁is ▁ 1 2 5   µ m ▁thick , ▁and ▁composed ▁of ▁dark ▁angular ▁to ▁roughly ▁spher ical ▁cells ▁that ▁are ▁ 4 0 – 7 0   µ m ▁in ▁diameter . ▁The ▁angular ▁cells ▁form ▁py ram idal ▁w arts ▁on ▁the ▁outer ▁surface . ▁The ▁med ull ary ▁ex cip ul um ▁( the ▁inner ▁f les hy ▁layer ▁of ▁t issue ▁under ne ath ▁the ▁e ct al ▁ex cip ul um ) ▁is ▁almost ▁gel atin ous , ▁composed ▁of ▁inter w oven ▁hy pha e ▁ 1 0   µ m ▁in ▁diameter . ▁ ▁Several ▁struct ural ▁components ▁are ▁involved ▁in ▁sp ore ▁dis charge ▁in ▁W .   amer icana , ▁such ▁as ▁the ▁as cus , ▁the ▁oper cul um , ▁the ▁sub oper cul um . ▁The ▁sp ore - b ear ing ▁cells , ▁the ▁as ci , ▁are ▁ 3 3 0 – 4 0 0   µ m ▁long ▁by ▁ 1 6 – 2 0   µ m ▁wide . ▁The ▁as cus ▁has ▁a ▁thick ened ▁ap ical ▁ring ▁that ▁is ▁c apped ▁by ▁a ▁h ing ed ▁oper cul um , ▁a ▁lid ▁that ▁is ▁opened ▁when ▁sp ores ▁are ▁to ▁be ▁released ▁from ▁the ▁as cus . ▁The ▁presence ▁of ▁the ▁ap ical ▁ring ▁beneath ▁the ▁oper cul um ▁and ▁the ▁sl anted ▁opening ▁that