344 
Races of Entamoeba histolytica 
that every sanij^le of faeces arriving at the laboratory has been passed by 
the particular patient whose name it bears. In spite of every precaution, 
substitution and interchange of specimens from time to time occur; and 
a case which is in reality negative may thus appear positive, and vice 
versa. On three occasions our patients have “ relapsed ” after treatment 
with emetine bismuth iodide, the “relapse” strain having cysts of a 
different size from those of the original infection. As such an occurrence 
appeared to us highly improbable in the light of the results given above, we 
were led on each occasion to challenge the authenticity of the specimens. 
A more rigorous control of the patients, and the continued examination 
of their faeces, ultimately proved in each case that our suspicions were 
jXIstifled. All three cases were subsecjuently found consistently negative, 
and were therefore Anally discharged as cured. Had we irot carefully 
studied and recorded the size of their cysts before treatment, each of 
these cases would, of course, have been wrongly classified as a relapse 
and re-treated. 
Comparison op our conclusions with those of some other workers. 
Before discussing our results and their relation to those of certain 
other observers, it will be convenient rapidly to survey these results 
themselves. We have, indeed, already summarized them to some extent 
in our previous paper (1917), and at various points in the course of the 
present work. But it will greatly facilitate the ensuing comparison if we 
again summarize our chief conclusions at this point collectively. 
Our chief result is that we have, as we believe, demonstrated that the 
human dysentery amoeba {E. histolytica Schaudinn) is a collective species, 
which includes a number of distinct races or strains of individuals which 
can be readily distinguished from one another by the sizes of the cysts 
which they produce. The cysts produced by the different strains difier 
from one another morphologically in no respect save size, being in all 
other respects indistingxxishable. Whether these strains differ from one 
another in pathogenicity or in other physiological characters we have as 
yet no means of determining with certainty. Nevertheless, we have 
observed that all react in the same way towards treatment with emetine, 
and we think it probable that no such differences exist. We believe that 
we have been able to show with considerable probability that the differ¬ 
ent groups of individuals constituting these so-called “strains” really 
are such, since they appear to be permanent and not to be merely a 
result of differing environmental conditions. In other words we regard 
them as constituting so-called “petites especes” (Jordan) or “pure lines” 
