915 
(b) Ectoplasm.—Density. Tenacity and dictinetion from endoplasm. 
Color and refraction. Contents, granules, and other elements. 
(c) Hndoplasm.—Presence of, absence of, or variation in size and 
number of granules. Color and refraction. Presence or absence of 
red blood corpuscles and other formed, foreign elements. Number, size, 
and contractility’ of vacuoles. Spore-appearing bodies. 
(d) Nucleus.—Location, shape, size, and variations in the amount of 
chromatin. Presence or absence of nuclear membrane and nucleolus. 
In order to obtain the most complete data, it is necessary as far as pos- 
sible to study the above characteristics in all stages of the life cycle of 
the parasites. 
(d) Amebe as a whole—Size.—The dimensions of amcebe vary be- 
tween wide limits, but probably from 3 to 40 » in diameter would be the 
minimum and maximum of size in cultures, and this variation is almost, 
or quite equalled when the parasites are observed in stools and in the 
tissues with ameebic lesions. In the naturally encysted stage, the varia- 
tion is much less, being from about 8 to 30 or perhaps 35 ». The expla- 
nation of the differences in measurements between amcebee in the encysted 
and in the vegetative stages is as follows: 
All amoebee are smaller when they are encysted than when in motion. 
The smallest sizes, often seen in the vegetative stage—that is, the ones 
below 8 to 10 » in diameter—are almost surely young parasites, and 
these do not naturally encyst until they are older and larger. Under 
certain adverse conditions, such as an extreme lowering of temperature, 
even the smallest and youngest forms may be driven to assume a cystic 
appearance, and, of course, under such circumstances they are exceedingly 
small. However, this change is not a true cyst formation, but micro- 
scoptcally it may be impossible to distinguish from real encystment. 
Formerly the variation in size of amcebe was considered to be of 
importance in determining species, and especially in separating the so- 
called pathogenic from the non-pathogenic types. However, more recent 
writers have attributed less importance to it, contenting themselves with 
the statement that “H. histolytica” is in general larger than “H. colt.” In 
our former work we have remarked that measurements of the parasites 
could certainly not be used in the determination of species. However, 
after more extended experience, it begins to appear as if measurements, 
if they could be made with parasites of equal age and in the same environ- 
ment, might become an important factor in determining species. ‘The 
trouble, of course, would be the difficulty of being sure of the age and 
environment of the amcebe. 
With cultures grown from a single amoeba, and hence pure, the indi- 
vidual examples vary much in size, particularly in the vegetative stage. 
In the encysted stage this variation, although present, is much less when 
amcebe on the same plate culture are considered. Between one culture 
and another of the same amoeba there may be a greater difference, 
