PARASITES OF SIMULIUM LARVAE 69 
towards the periphery; the protoplasm meanwhile becomes more 
condensed on the side of the vacuole which is farthest from the 
periphery of the sporoblast. At the same time the cell assumes 
a more elliptical form and its superficial layer becomes differen- 
tiated into a very transparent thick shell. 
Spores. The sporoblasts have now been transformed into 
spores. These are short elliptical bodies, somewhat truncate at 
the ends, measuring about 3u x 2.5 — 2.7y (pl. 2, figs. 14 to 17 and 
21 to 24). They still remain attached to one another in aggre- 
gates of eight, although they are not in any way actually united | 
with one another. In size and shape they are extremely uni- 
form, though I have observed two larger spores measuring some 
8 x 5u. As the occurrence of such forms is extremely rare I am 
inclined to consider that their presence is due to some abormality 
in development. In a fresh state the spores show no differentia- 
tion except a small refractive area near one extremity. This is 
the vacuole. If weak iodine solution be added to the water 
in which the spores are floating the inner wall of the very thick 
shell can usually be seen, while in a very few cases a comparatively 
stout filament, some six times the length of the spore, is extruded 
from the end of the spore farthest from the vacuole. Although 
undoubtedly all the spores are capable of protruding this filament 
I was able to cause them to do so in considerably less than one 
out of every thousand cases, though I tried many of the usual 
reagents for the purpose. From a nuclear study I am inclined to 
believe that the spores were not quite ripe and that this accounted 
for my inability to extrude the filament in more cases. As 
winter approached development evidently ceased, and spores 
dissected out of larvae and kept in water showed no signs of con- 
tinued development. 
For studying the internal structures of the spores the most 
successful stain used was that of azure II and eosin A. B. as 
previously described. In the young spore the nucleus occupies 
the extremity of the cell opposite to that occupied by the 
vacuole. At first it is rather diffuse but later becomes more 
concentrated to form a small globular body (pl. 2, fig. 14). 
Meanwhile the cytoplasm becomes still further condensed till 
