The terminology proposed by these authors is based on the terminology by INVAM and Stürmer & Morton (1999). In this terminology, discrete structures that are easily separated when a spore is broken are named to its relative position (outer, middle and inner walls).
1) Outer wall: it corresponds to the Spore wall in the terminology of Morton et al. (1995) and to Group A in the terminology of Walker (1983).
2) Middle wall: it corresponds to the Germinal wall 1 in the terminology of Morton et al. (1995) and to Group B in the terminology of Walker (1983).
3) Inner wall: defined as the one where spore germination starts. It corresponds to the Germinal wall 2 or 3 in the terminology of Morton et al. (1995) and to Group C or D in the terminology of Walker (1983).
References:
Sieverding, E. & Oehl, F. 2006. Revision of Entrophospora and description of Kuklospora and Intraspora, two new genera in the arbuscular mycorrhizal Glomeromycetes. J. App. Bot and Food Qual. 80:69-81.
Morton, J.B., Bentivenga, S.P. & Bever, J.D. 1995. Discovery, measurement, and interpretation of diversity in arbuscular mycorrhizal fungi (Glomales, Zygomycetes). Can. J. Bot 73:
Walker, C. 1983. Taxonomic concepts in the Endogonaceae: spore wall characteristics in species descriptions. Mycotaxon 18:443-445.