by the Method of Dark-ground I Humiliation. 605 
cover-slip and the liquid drawn under by placing a piece of filter-paper 
along the other edge. The method necessarily brings about a dilution of 
the solution, so that no critical knowledge of the concentration of the reacting 
solution is possible. The other method is to mount the objects directly in 
the solution and to observe the effects as soon as possible. This is suitable 
for actions with a considerable time factor, such as plasmolysis, but is useless 
in studying the first stage of fixation and coagulation. 
The figures are drawn to illustrate as nearly as possible the appearance 
of the objects. The main outlines were sketched with a camera lucida, the 
smaller details being put in as accurately as possible. In many cases it is 
impossible to represent the smaller particles which are present, and in any 
case the drawing, lacking animation, gives no real idea of the activity of the 
particles of the hydrosols. 
§ 3. Observations of Living Cell Structures. 
A description will first of all be given of some of the material which 
has been examined in the normal living state by this method and sub- 
sequently used for other experiments. Some of the objects here described 
were observed by Gaidukov (TO), but in these cases further facts as to 
structure have been made out, while the others are here described for the 
first time. 
Spirogyra. 
Spirogyra has been carefully observed and described by Gaidukov, 
and furnished one of the most favourable objects. This is fortunate, as it 
has so often been used by investigators on plasmolysis and the physiology 
of the single cell. A few further facts as to its colloid structure may be 
added. 
According to the nature of the study, types with closer or more distant 
chloroplast spirals may be chosen. By focusing outside the chloroplast, it 
is clearly seen that comparatively large microsomes exist in the protoplasmic 
hydrosol — at least these appear large with this method of illumination. In 
certain cases some of these microsomes can be made out in direct illumina- 
tion ( Hellfeldbeleuch tnng ) . As will be shown more fully below, a large 
number of much smaller particles occur in the protoplasm, which can often 
be made out by careful focusing and manipulation of the light. From these 
careful focusing experiments, it appears that the smaller particles tend To 
occur more especially towards the outside and inside of the plasma layer 
(Price, T 2 ). It may be mentioned here also, that the chloroplast has 
a structure very distinct from that of the cytoplasm. No distinct micro- 
somes appear in it, and on the whole it seems to have a fairly homogeneous 
gel-like structure. In reaction, as shown below, it differs quite markedly ; 
the subject will be referred to again (see § 7). 
