Hydrodictyon utriculatum . 625 
there are some important structural changes to be noted. 
The dense homogeneous structure becomes more spongy, 
with denser and less dense regions. There is, however, no 
regularity in the form or distribution of such regions. Fre- 
quently there is simply a dense region toward the outside 
with the less dense toward the unchanged part of the pyrenoid 
(Figs. 3 and 8). Very often, as indicated above, the denser 
regions may be so distributed as to give the appearance of 
a roughly alveolar structure (Fig. 6). In any case the denser 
regions soon become more prominent and take up the blue 
stain. They apparently increase in size until nearly the 
whole part of the pyrenoid undergoing transformation presents 
a deeply blue stained homogeneous structure, the typical 
appearance of all the starch-grains in this plant. There is 
nearly always between the fully formed starch-grain and the 
unchanged remainder of the pyrenoid a thin zone of slightly 
stained material (Figs. 1 and 6), as if with increasing density 
there were a shrinkage in the whole mass. Such a zone may 
appear before the starch-grain has reached the homogeneous 
character of the mature grain. There is in effect a splitting 
off of a segment of the pyrenoid which is metamorphized 
into a starch-grain. When the grain of starch is fully formed 
it appears to lie in a vesicle or vacuole in the cytoplasm, but 
without being surrounded by a differentiated membrane. The 
vesicular appearance may of course be due to a slight con- 
traction of the protoplasm in fixing, but this is a point that 
could not be settled definitely. 
The mature grain has practically the shape of the segment 
of the pyrenoid from which it is formed. Even after it is 
widely separated from the pyrenoid, in the manner described 
below, the plano-convex or concavo-convex form is retained 
(Fig. 1). This is a point of much importance, in establishing 
the fact that all of the starch is formed from the pyrenoid. 
When starch is being formed rapidly a second grain will 
be built at once before the pyrenoid regains its original form. 
The remaining part of the pyrenoid undergoes the same 
differentiation as was seen in the original body, and the 
