248 
W. B. Mercer, 
cannot be seen may usually be found among and outside the loose hypliae 
(lower part of Fig. 1, /^); the central mass also begins to assume a 
yellowish colour. As development proceeds the central mass enlarges, the 
surrounding network of hyphae is to a great extent absorbed, and the 
peripheral cells become sharply differentiated, their colour changing through 
pale yellow and brown to black. 
Meristogen. 
This method of for¬ 
mation is confined 
to late formed pye- 
nidia. Development 
is much slower than 
in the former case. 
In moist chambers 
extremely few prim- 
ordia arise in this 
manner, and these 
rarely develop into 
pycnidia. Though the 
earlier stages in the 
evolution of a num¬ 
ber were followed in 
the same manner as 
for the symphyogen, 
no case was met with 
which completed its 
development. Evo¬ 
lution, therefore, has 
been traced by a 
comparison of exam¬ 
ples in different sta¬ 
ges of development 
occurring in moist- 
chamber and Petri- 
Fig. 1 . i Mycelium on plum agar. — 2 , j: Fusions of 
hyphae. — 4 : Old hyphae with large oil drops. — 5 , 6 : 
Isolated stretches of living hyphae in old mycelium. — 
7 : Mycelium on Salep agar. —8 : Mycelium on Leci¬ 
thin-gelatine. — 9 : Mycelium on Potato. — 10 : Pyc- 
nidium on surface of sterilized Calla-\eai. — //: Section of 
upper part of same. — 12 — 14 -. Evolution of symphyogen 
pycnidium on plum agar. — 15 — 21 : Devotoping meri¬ 
stogen pycnidia on Salep agar. — 22 — 26 : Pycnidia 
developing from pycnospores in Cane Sugar solution. — 
27 — 2g : Pycnidia developing by intermediate methods. — 
Fig. 1—8 = 3B;1 / 1 , Fig. 9—29 = 263 / r 
dish cultures. 
The main part 
of the primordium is 
derived from the re¬ 
peated division and 
growth of a few ad¬ 
joining cells of a 
liypha. At a particular 
point in a hypha a 
number of cross walls 
are laid down at close 
interwals (Fig. 1, /j). The short cells so formed swell and divide in all 
directions; the daughter cells repeat the process, a solid cellular mass 
being formed (Fig. 1, 16 , if). A certain amount of branching usually 
takes place, the branches being small and divided into short cells which 
fuse with the cells derived from the division of the main hypha (Fig. 1, 
iS — 20 ). The exact amount of assistance the growing mass receives by 
