by the Method of Dark-ground Illumination* 613 
to regard the whole mass as protoplasmic, with the nature of an emulsion 
colloid, the globules, of whatever nature they may be, forming the disperse 
phase, in a possibly complex hydrosol basis — probably not a true continuous 
phase. 
§ 5. The Sap Particles or Inclusions. 
Solid particles have long been known to occur in the cell sap in certain 
cases, where they execute an oscillating Brownian movement, the best- 
known case being the particles of calcium sulphate and so on, which are 
present in the terminal vacuoles of some Desmids (West, W. and G. S., ’04). 
They have also been frequently noticed, but not very definitely recorded, as 
occurring in the cells of many Green Algae. Professor West kindly informs 
me that he has observed such small particles in the cells of Spirogyra , 
Zygnema , Microspora , Tribonema , Ulothrix , and many Desmids and others, 
and he regards them rather as a sign of approaching death, or an indication 
of bad health of the filament. 
As already stated, sap particles have been observed by the dark- 
ground method in many, or indeed most, of the cells so far examined. Any 
small particles of a solid nature free in the cell sap will perform a Brownian 
movement as these particles do. 
They are generally rendered conspicuous as bright specks or definite 
outlines, of various sizes, with an oscillatory and translatory motion certainly 
of a Brownian nature, the rate of this varying generally with the size of the 
particles. The particles are almost certainly to be regarded as of different 
nature in different cases ; they certainly vary in form and appearance. 
Generally, those so far encountered seem to fall into two categories of form 
as follows : 
(a) Structures or particles of a vesicular or drop-like nature, with 
a higher refractive index than that of the cell sap. 
(b) Particles with a regular and apparently prismatic crystalline form ; 
at least this is the idea given by them as they pass through different 
positions. PI. XLII, Fig. 6, indicates a series of diagrammatic representations 
of such particles from the leaf cells of Elodea drawn in different positions. 
They thus appear to be flat rectangular plates of a transparent crystalline 
nature. Little idea of their shape could be gathered in direct illumination, 
and no results were obtained by the use of polarized light or crossed nicols. 
With regard to class (a), probably different forms are included here, as 
there seem to be particles with a solid structure, as well as others of the 
nature of bubbles or vesicles in the liquid. It is generally very difficult to 
distinguish the two, however. 
It seems probable that they have been noticed before only in the more 
obvious cases, and when fairly abundantly present. Also it appears certain 
that they increase greatly in numbers and in size as the cell approaches its 
T t s 
