C. Dobell 
183 
Isospora bigemina var. hominis (Raill. et Luc.) Fantham, 1917. 
Coccidium isospora Savage et Young, 1917. 
Coccidia (isospora), Martin, Kellaway et Williams, 1918. 
Coccidium (Isospora sp.) Boney, Crossman, et Boulenger, 1918. 
(See PI. VIII, fig. 1, and Text-fig. 1, B, p. 177.) 
The history of our knowledge of this parasite has already been dealt with 
in some detail. So far as I can judge it was originally discovered by Kjellberg, 
about 1860, in the villi of the small intestine. It was then seen—probably, but 
not certainly—though not named, by Eimer (1870); and first named, but not 
seen, by Rivolta (1878). Railliet and Lucet (1890) possibly found its oocysts 
in the faeces for the first time, but this is somewhat doubtful; and Rivolta and 
Grassi certainly did not find them, though often alleged to have done so. The 
first correct and clearly recognizable account of the oocysts and spores is that 
of Wenyon (1915, 1915 a). Drawings of these have been published by Wenyon 
(1915 a, 1915 c)—the latter work containing also some good photographs of 
the oocysts, taken by Dr A. C. Stevenson. Since the appearance of Wenyon's 
description over fifty cases of infection with this organism have been re¬ 
corded. 
The schizogonic cycle of development in the intestine has yet to be in¬ 
vestigated 1 . It is doubtless closely similar to that of I. bigemina in the dog and 
cat. The organism is accurately known, up to the present, only from its 
oocysts in human faeces. They are elongate ovoid in form, the narrow end 
being usually somewhat drawn out into a sort of neck. They vary in length 
from about 25/x to 33/x; their width, at the widest part, being about half their 
length (ca. 12-5/x to 16/^). The relative dimensions of the oocysts are not 
constant, however, though they are much oftener long and slender than short 
and plump. (See Plate VIII, fig. 1, and Text-fig. 1 , B.) The oocyst wall (oocyst 
proper) is clear, colourless, and porcellaneous in appearance. It consists of at 
least two—and probably more—layers, and is impervious to most fixing fluids. 
A micropyle is probably present at the narrower end of the oocyst, but it is 
inconspicuous. 
The oocysts of /. hominis , like those of I. bigemina , are usually discharged 
from the body in an unsegmented condition (cf. fig. 1, PI. VIII), though 
occasionally oocysts already containing two sporoblasts may be found in the 
freshly passed faeces. Development of the spores takes place outside the host, 
and requires several days for its completion. In this respect Isospora hominis 
differs conspicuously from the two species of Eimeria to be next described. 
At the time of its ejection from the body of the host, the oocyst generally has 
its protoplasmic contents contracted into an almost spherical mass, filled with 
brightly retractile granules. A clear area amidst these is often discernible, and 
probably represents the nucleus. (Cf. fig. 1, PI. VIII.) 
1 The statements concerning the schizogony, fertilization, etc., of this form, made by Wen von 
(1915 a), are presumably inferences from what is known of the development of allied forms. 
