1887. ] MICROSCOPICAL JOURNAL. 103 
It is very doubtful if there be any envelope differing essentially from the 
protoplasmic substance of the corpuscle which envelopes the substance. On 
the other hand it seems most probable that the substance is naked, that it is 
not covered by any envelope or cell-wall. If one examine the living blood 
corpuscle under certain favorable conditions it will be seen to undergo 
changes of shape, and from the globular assume stellar forms, ‘ amaboid 
forms,’ and this indicates that a wall of any great density could not be pres- 
ent. Moreover, in examination of the corpuscle no wall can be seen separate 
from the protoplasm as may be seen in many other cells. 
We must note before leaving the blood corpuscle how the microscope 
alone is unable to furnish all the facts which the biologist must possess. 
The corpuscles are a part of the blood, but the other part, the fluid which 
carries the corpuscles, is not seen though, we doubt not, present in the blood 
space (b. c.) and to be studied in other ways. As a study of the blood itself 
would form a topic for a special chapter, I must leave it for the present. 
c. The basement membrane.—The blood space (b. c.) is bounded, 
except upon the outer borders of the gland, by the walls of the cavities of the 
glands called the ‘ a/veolz.’ The alveolus of the gland is the active portion 
of the apparatus in vertebrate language, and we may use the same term 
here, though here duct and alveolus are less different in structure than in the 
salivary gland of the cat, for instance. The wall is composed of cells packed 
closely together and they are supported by a very thin and delicate structure 
which also bounds the blood space, the ‘basement membrane.’ This mem- 
brane is not always to be seen inall parts of the section, and, in fact, more 
often it cannot be seen. Looking at the borders of the blood space, one most 
often finds the end of the cell apparently in direct connection with the space, 
but in numerous places a very sharp though very thin line may be discovered 
between the cells of the alveolus wall and the blood space. 
Other investigations show with regard to the basement membrane that itis it- 
self cellular, though sections across it ordinarily show no trace of a cellular 
structure. {t is composed (in mammals) of very broad and flat cells, spread 
out to form a surface on which the cells of the alveolus wall are carried. A 
basement membrane thus carrying cells is, together with the cells carried, 
called an epithelium, and the green gland of the cray-fish gives us an intro- 
duction to an epithelium in its simplest form. One point in regard to the 
position of the basement membrane requires especial emphasis, for, while 
characteristic of epithelium in all animal histology, it is seen here illustrated 
in the simplest manner. It is that the basement membrane always stands be- 
tween the blood spaces (or blood vessels in the higher animals) and the epithe- 
lium cells, and forms an impassable barrier to all substances except those es- 
pecially provided for, which would pass to or from the blood through it. 
Since the basement membrane is very important for the working of animal 
bodies, its presence as part of every epithelium surface should be demon- 
strated if possible. But it is often so difficult to see that a prolonged search 
may be necessary along the walls of the alveoli before a place is found where 
it can be distinctly demonstrated. Such a search will usually be rewarded, 
and its presence, clearly in a few places and faintly in others, is evidence of 
its general presence, despite the difficulty of seeing it elsewhere. The base- 
ment membrane bounds what is known as the inner ends of the epithelium 
cells. 
_ d. The outer cell-wall will be seen approximately parallel with the 
basement membrane. In the part we are now particularly describing (fig. 3) 
it is a very faint line intervening between the lumen of the alveolus (Lu) and 
the substance of the cell. It is one-thousandth of an inch, or more often 
less, from the basement membrane, and is the limiting membrane or wall 
