124 THE AMERICAN MONTHLY [July, 
As the three kinds of cells were found to vary in shape and size, so one 
can see a difference between the three in the character of their contents. In 
all of them the ‘ cell substance,’ so far as we can see, is protoplasmic. 1. In 
the cubical cells the protoplasm is evenly distributed through the cell, except- 
ing under the outer wall, where it shows a deeper color, as if it were a 
trifle denser there than through the rest of the cell. 2. In the columnar cells 
there is a very marked difference, the protoplasm here is heaped up in the 
inner end of the cell and is very thin in the outer end ; thus the wall made up 
of these cells exhibits a sort of double band of inner deep color and outer faint 
color. 3. Inthe flat cells the cell substance is very much lessgranular. It has 
a very deep color and a semi-translucent appearance with bright transmitted 
illumination. This is not well shown in the figure, but is very conspicuous in 
the section, making it possible to pick out this sort of lining at once with a 
very low power. 
In addition to the protoplasmic contents spread through the cell a proto- 
plasmic body remains to be described— 
The nucleus.—This is plainly seen in many places, more plainly often 
than the outline of the cell to which it belongs. In studying it it is impor- 
tant to observe its shape, size, and position in the cell also, so much of its 
finer structure can be determined. 
Inthe alveolar cells the nucleus presents an outer sharply-defined boundary 
wall, which runs around it as a thin, sharp line. Within this line a faintly 
granular mass is spread about, and here and there in this mass several very 
dark spots are to be seen. One of these is usually central in position, while 
the others lie close to the boundary wall. The nuclei in the cubical and col- 
umunar cells lie in the centre, or rather nearer the inner end of the cell, and 
surrounded by the protoplasm. ; 
d. Arrangement of the cells.—Having traced the course to be pursued 
to prepare and study the green gland, and to study its histological or cell 
structure, so far as relates to certain of the cells which are found in it, we 
have further to see how these cells are combined, and, if possible, to deter- 
mine how the entire organ is built. It is time now to bring forward the name 
‘tissue’ for these aggregates of similar cells, like figure 2, for example; and 
in their case a very simple example of a tissue is presented. As in the case 
of a single cell, we have only one slice and one view, so in the case of the 
whole organ, to which we now return, we have but a single slice. Figure 1 
is a Somewhat simplified representation of such a slice. Our task is to learn, 
if possible, how the individual cells go together to form the whole structure. 
A study of the section will show that the ‘ outer ends’ of the epithelium cells 
always face toward the cavity of which they help to form the wall. The inner 
end of the cell, on the other hand, is always farthest from the cavity or lumen, 
and beyond it still is the basement membrane. Thus, e..2., fig 3: Zaz stands 
in the cavity of such a place as & in fig. 1. This is true in every instance, as 
the observer can verify upon his sections, though not on figure 1, where the 
difference between the inner and the outer ends of the cells is not shown. It 
is not only true of the cells of the outer wall, which bounds the whole cavity, 
but it is true of certain islands of tissue, which (in the sections) have no con- 
nection with the outer boundary wall. Its cells, upon examination, present 
an outer end and an inner end, with a basement membrane farthest from the 
lumen of the alveolus. Sometimes blood corpuscles are to be seen surrounded 
by these islands of tissue ; but it is to be noted that in every case the basement 
membrane intervenes between the blood corpuscles and the cells. Wherever 
the epithelium comes out upon the outer edge of the gland, as at H, for in- 
stance, it is the inner ends of the cells which are nearest the surface of the 
glands, while the outer ends of the cells are turned toward the lumen of the 
