1906] ATKINSON—AGARICUS CAMPESTRIS 251 
the surface, showing that the pileus primordium is becoming definitely 
organized through the central portion (fig. 4), also sometimes seen 
in freehand sections in older stages as a compact area (fig. T5). 
In the very early stage of the hymenial primordium the under 
surface is even, but very soon the outlines of the gills begin to form 
by the more rapid downward growth in radial lines. Very soon 
after the young gills begin to form, the surface of the stem is differen- 
tiated, This occurs in such a way as to show that the surface of 
the stem portion of the young fruit body does not become the surface 
of the stem except at its extreme lower portion, which probably cor- 
responds to the bulb even when a bulb is not manifest as a thicker 
portion of the stem. From a point at the junction of the original 
annular primordium of the hymenium, or the junction of the young 
stem with the pileus at the inner end of the gills, and then extending 
obliquely downward and outward, the hyphae take on more active 
intercalary growth and a richer content of protoplasm. This marks 
the origin of the cortex of the stem, and distinguishes it very sharply 
from the elements of the veil and from the internal tissue of the stem 
which is to be the medulla by a deeper color in stained preparations; 
also in fresh freehand sections it is usually very clearly seen with the 
unaided eye asa whitish more compact area which shows well in 
- ordinary photographs at this stage. 
This study shows very clearly that the hymenium in Agaricus 
campestris is endogeneous in its origin, as HorrMann described so 
early as 1856 (J. c.), and that DeBary’s first account of the develop- 
ment of this species ia 1866 was correct so far as it went; but he did 
hot succeed in obtaining for study carpophores sufficiently young to 
enable him to speak with certainty after Hartic had thrown doubt 
on his conclusions as a result of his study of Agaricus melleus. 
Not only is the hymenium endogenous in its origin, but the differen- 
tiation of the stipe and pileus is simultaneous; that is, the initial 
Stages of the stipe and pileus as distinct structures are organized and 
made evident in longitudinal sections by the origin of the hymenial 
Primordium, In all the sections that I have examined of this species 
at this Stage, there is no evidence of differentiation of the pileus 
from the stem before the earliest evidence of the hymenial. primor- 
dium. We should not conclude, however, this mode of development 
