1. The basidiocarp primordium consists of slender hyphae intricately interwoven; they are arranged more loosely in the peripheral region, and radiate from the entire surface of the fruit body. 2. The blematogen is present from the first differentiation of the fruit body primordium, and in its earliest stages consists of the loose, radiating peripheral hyphae. In subsequent growth it forms a thick layer enveloping the entire plant; in P. flammans and P. adiposa it becomes partially disorganized by gelatinization. 3. The formation of the stem fundament is the first differentiation to take place in the young fruit body. It originates in the basal part of the basidiocarp and by growth and differentiation progresses toward the apex. 4. The primordium of the hymenophore is differentiated around the apex of the stem fundament as an annular internal zone of new growth. Frequently, before the hymenophore appears, a slight divergence of hyphae from the stem apex indicates differentiation of the pileus. When the hymenophore primordium is differentiated, it marks off clearly the limit between the pileus and stem. It consists of slender hyphae, rich in protoplasm, which grow downward. At first the lower surface is uneven and loose, but by continued growth the hymenophore becomes compact and the hyphae grow down to the same level, forming an even palisade area. The growth of the hymenophore and organization of the pileus proceed centrifugally. 5. The annular prelamellar cavity is formed by the tearing away of the ground tissue from the lower surface of the hymenophore, due to the tension exerted by the growth and expansion of the plant parts. It is weak in P. squarrosa, and well formed in P. flammans and P. adiposa, before the origin of the lamellae. 6. The marginal veil consists of two layers; the inner portion is composed of the ground tissue left in the angle between the stem and the hymenophore after their differentiation, which increases by its own growth and by the addition to it of hyphae from the pileus margin. The outer portion is a section of the universal veil between the pileus margin and the surface of the stem. 7. The lamellae originate as a series of radiating areas of active hyphae in the hymenophore, which grow down and push the palisade layer into folds. The points of growth for the origin of the gills are in these downward growing areas in the hymenophore, and the first folds in the palisade are the salients of the lamellae themselves. In P. flammans and P. adiposa the gill salients are very broad; in all 3 species their origin and differentiation is centrifugal and their subsequent growth is downward into the gill cavity.