The propagation of single flat lamellae
into three dimensions occurs at screw
dislocations, where one lamella divides
into two, which grow apart at a small
angle. In polyethylene (and some other
polymers) these screw dislocations
form regularly and with the same
hand, so that the daughter lamellae
formed on outward growth do so at a
continually increasing angle. But this is
an incremental process, not a
continuous spiral.