D. Keilin 
105 
After differentiation in iron alum, the darkly stained axial portion of the 
filament presents a granular structure, and if the differentiation is more 
prolonged, until the filament becomes of a grey colour, a darkly stained 
chromatic body, of definite size and structure, makes its appearance (Text- 
fig. 4). This body, which is undoubtedly a nucleus, is 2-3/x long and 0-6 p wide, 
always lies in the wider portion of the filament, 15-18/x from its extremity. 
By prolonged differentiation this nucleus can also be detected in filaments of 
the spiral form while still enclosed in the sporocyst (PL IY r , fig. 41). We 
now arrive at the final stage of the life-cycle of the parasite: the distended 
cuticle of the dead larva being filled with a felt-like mass of entangled filaments 
mixed with small cells, the central cells of the spores and the empty sporo- 
cysts (PI. V, figs. 8 and 9). 
At this stage the macerated cuticle of the larva breaks away and its entire 
contents escape into the surrounding medium, the decomposed sap of the 
tree. Smear preparations of this sap often reveal the above-mentioned fila¬ 
ments, while the central cells of the spores cannot be recognised in the crowd 
of various micro-organisms, yeasts, moulds, rhizopods, ciliates, etc., which 
usually inhabit the fluid. 
5. Supposed mode of infection of the host. 
A question arises now: which is the infective form of Helicosporidium'l Is 
it the filament or the central cells of the spore? The cellular structure of the 
filament, its great resistance to external influences, and its resemblance to the 
infective stage of Monosporella suggest that it may be the infective stage. 
If this is the case, the central cells of the spore could only be considered as 
residual bodies. 
On the other hand, the great number of the central cells (three times 
more numerous than the filaments) and their resemblance to the first stages 
of Helicosporidium, as seen in the body-cavity of the host, make it almost 
certain that they represent the real sporozoites or infective forms of the 
parasite. In this case the spiral filament may be regarded as a cell differen¬ 
tiated for the purpose of dehiscence of the spore and can be compared in 
respect to its function with the elaters of Mycetozoa, with the difference that, 
while in the latter they are of complicated structure and extrasporal, in 
Helicosporidium they are unicellular and intrasporal. 
The sporozoites, after being swallowed bv a healthy larva, penetrate 
probably through the wall of the alimentary canal into the body cavity of 
the larva, where they begin their endogenic multiplication or schizogonic cycle. 
6. Frequency of infection of Dasyhelea larvae. 
Several generations of Dasyhelea obscura occur in the course of a year, 
and the larvae of all the generations are equally subject to infection. It is, 
however, impossible to estimate the true rate of infection as this varies 
greatly and depends upon the condition of the wound of the tree at the time 
