DEVELOPMENT OF EXOGENOUS SPECIES OF AGARICS 
39 
the appearance of figures 6, 17, 23. Compared with the thickness of 
the elements, this paHsade layer is an unusually deep one. The hyphae 
become more and more crowded as a result of their rapid branching 
and their slight increase in diameter. For this reason the layer 
becomes very conspicuous in contrast to the looser, more inactive 
subhymenial tissue, from which these hyphae have arisen. 
Development of the Gills. — Fruit bodies in the stage shown in figures 
8-12 show the very earliest evidences of gill formation. We have 
seen that the palisade layer has become very crowded by the multi- 
plication of its elements. As these continue to be formed, this extra 
growth produces a tension which must be taken care of in some way. 
This is accomplished by the growth downward of the subadjacent 
hyphae in regularly spaced radial rows, beginning at the stem and 
extending to the margin of the pileus. In figure 8 the palisade layer 
appears to be slightly decurrent on the stem. It is here that the 
salients first begin to form. These appear as two irregular folds in 
figure 9, which is reproduced from a section cut parallel to the median 
plane at the surface of the stem. As one passes outward, these folds 
become less marked (fig. 10) and finally disappear altogether. In 
figure II, nearer the margin, we find the level palisade layer, while at 
the very outside of the pileus the irregular palisade primordium is 
present (fig. 12). The series of sections in figures 13-17, from a more 
mature plant, shows better-developed salients, while figure 18 repre- 
sents a section through the margin. These salients are becoming more 
regular and, as the pileus broadens and the interstitial growth forces 
the primary salients apart, new secondary ridges make their appearance 
between the original ones (fig. 21). This method of gill development 
is then, in the main, the same as that described by Hoffmann (17, 18, 
19) and DeBary (11, 12, 13, 14) in exogenous forms and also agrees 
with species recently studied by Blizzard (9). It is also similar to the 
method of gill origin in endogenous forms of the Agaricus type de- 
scribed by the earlier workers and in the modern work of Miss Allen (i), 
Atkinson (2, 3, 4, 5, 6, 7), Beer (8), Douglas (15), Sawyer (20) and 
Zeller (21), with the exception that in the endogenous forms, the 
ridges develop underneath in a more or less distinct ''gill cavity," 
while in the exogeneous forms they are exposed from the first to the 
outside. 
Development of the Pileus. — After the appearance of the annular 
furrow separating the pileus primordium from that of the stem, the 
