1906] ATKINSON—AGARICUS CAMPESTRIS 240 
looser and less dense arrangement. This layer is the “universal 
veil.” It is quite distinct in the young stages of these cultivated 
varieties, continues to increase in extent until the parts of the fruit 
body are differentiated and the young pileus and stem are manifest 
by external differences in form. Then it ceases to grow and is torn 
apart into white floccose patches on the pileus, as will be seen later. 
In the very young primordia then there is no evidence of a differen- 
tiation into stem and pileus in any of the many individuals which I 
have examined, As the primordia become slightly larger and older, 
but still before there is any evidence on the outside of an annular 
furrow or of any differentiation into pileus and stipe, longitudinal 
sections which are stained show two small deeply stained internal 
areas near the upper end of the young fruit body and some distance 
in from the surface. The hyphae here are not yet differentiated, 
but are richer in protoplasm, showing the origin of a new and special 
center of growth. This area is an annular one within the fruit body. 
Very soon afterward this area increases somewhat in extent and 
many hyphae begin to grow from the upper portion of this area 
downward. This is the primordial layer of the hymenium. It first 
arises when the tissue of the fruit body is homogeneous and compact 
except for the loose thin envelop. The hyphae which grow downward 
at this early stage are quite characteristic, They are slender and 
terete, tapering out into a long slender point. This enables them 
to pierce between the other hyphae of the compact fruit body. In 
fact at this time there are similar hyphae in the more central and 
upper portions, where the stem and pileus are to be differentiated, 
but in no other place at this time is there a definite center of growth 
which indicates the organization of any special part. ‘These hyphae, 
partly at least, provide for intercalary growth of the young fruit body. 
Soon after the hyphae in the primordial layer of the hymenium begin 
to grow downward, there is a cessation of growth just below this 
— which results in the rupture and separation of the hyphae at this 
point in a corresponding internal annular area, forming the well 
known “gill cavity,” which at first is very minute. 
_ This annular primordium of the hymenium marks the differentia- 
tion of the primordium of the fruit body into the primordium of the 
pileus and that of the stem and veil, the latter being the tissue of 
