10 
GEORGE F. ATKINSON 
DEVELOPMENT OF AGARICUS COMTULUS 
Primordia of pileus, hymenophore and stem. — The youngest fruit 
body of Agaricus comtulus Fr. sectioned was 1.25 mm. long by .75 
mm. in diameter. This shows the earhest stages of the primordium 
of pileus, stem and hymenophore, before there is any indication of the 
annular gill cavity. In longitudinal section the primordium of the 
hymenophore appears as two dense, deeply staining, rounded areas, 
symmetrically located on either side of the axis and some distance 
below the surface, showing that it is also endogenous in origin. The 
pileus primordium above is indicated by a very faintly staining dome- 
shaped area. Below, the stem primordium is indicated by a more 
densely staining area, distinct in this respect from the lighter staining 
basal area in which the tissue is of a looser texture. 
Character of the external zone of the young carpophore. — Above the 
pileus primordium, and lateral to the hymenophore, the tissue is of a 
much looser texture, the hyphae in general having a radial arrange- 
ment but flexuous and intricately interwoven with adjacent ones 
forming a loose mesh work, not at all comparable to the palisade layer 
of radial hyphae enveloping the upper part of the carpophore in 
Lepiota clypeolaria, Ar miliaria mellea, etc. 
Later stages of organization. — In the next stage examined the fruit 
body was 5X3 mm. The fundamental parts of the fruit body are 
well advanced and the external constriction marks the limit of pileus 
and stem. The gill cavity and hymenophore are already evident 
(figure 10). The rudimentary hymenophore is furnished with the 
palisade layer of blunt cells. The tissue which forms the partial veil, 
lying between the annular gill cavity and the outside is already of 
much looser texture, the lower portion of a much more open mesh, 
while the upper portion next the hymenophore or gill cavity is more 
dense as described above for Ag. arvensis. 
In figure 1 1 is represented a much older fruit body where gill 
formation has already reached quite an advanced stage. The outline 
of the stem surface is very clearly shown. The duplex character of 
the veil is very striking with its looser texture below and the denser 
portion above. The further stages of its development are as usual. 
THE UNIVERSAL AND PARTIAL VEILS IN AGARICUS 
In the three species of Agaricus, which I have thus far studied, 
the hymenophore is of deeper origin than that of Psalliota rubella 
