1984, D. E. Harder, 11: Developmental Ultrastructure of Hyphae and Spores, William Bushnell, Alan P. Roelfs (editors), The Cereal Rusts: Origins, Volume I: Specificity, Structure, and Physiology, page 355, In the inner wall facing the aecial cavity, the primary wall materials surrounding the processes disintegrate, partially exposing them.
Source: wiktionary