d around the copula (zygote), which effectually
excludes other less fortunate ones. This membrane rapidly increases in
thickness and becomes the oocyst (IX.), and the copula is now ready to
begin sporogony.

Sporogony goes on indifferently either inside the host or after the cyst
has been passed out with the faeces to the exterior. The definitive
nucleus of the zygote (resulting from the intimate fusion of the male and
female pronuclei, by means of a somewhat elaborate "fertilization-spindle"
[X.]) gives rise by successive direct divisions to four nuclei (XII.),
around which the protoplasm becomes segregated; these segments form the
four sporoblasts. Around each sporoblast two membranes are successively
secreted (exospore and endospore), which constitute the sporocyst (XIII.);
the sporocyst and its contents forming the spore. The nucleus of each
spore next divides, again directly, and this is followed by the division
of the cytoplasm. As a final result, each of the four spores contains two
germs (sporozoites), and a certain amount of residual protoplasm (fig. 3,
XIV.); this latter encloses a viscid, vacuole-like body, which aids in the
subsequent dehiscence of the sporocyst. On being eaten by a fresh host,
the wall of the oocyst is dissolved at a particular region by the
digestive juices, which are thus enabled to reach the spores and cause the
rupture of the sporocysts. As the result of instructive experiments,
Metzner has shown that it is the pancreatic and not the gastric juice by
which this liberation of the germs is effected. The liberated sporozoites
creep out and proceed to infect the epithelial cells. The sporozoites
(XV.) are from 15-20 [mu] long by 4-6 [mu] wide; they are fairly similar to
merozoites in form, structure and behaviour, the chief point of
distinction being that they have no karyosome in the nucleus (cf. above).

PLATE I.

[Illustration: FIG. 1.--SECTION THROUGH RABBIT'S LIVER, INFECTED WITH
_COCCIDIUM CUNICULI_. (AFTER THOMA.)]

[Illustration: FIG. 2.--_KLOSSIA HELICINA_, FROM KIDNEY OF HELIX
HORTENSIS.

a, Portion of a section of the kidney showing normal epithelial cells
containing concretions (c), and enlarged epithelial cells containing
the parasite (k) in various stages; b, cyst of the _Klossia_
containing sporoblasts; c, cyst with ripe spores, each enclosing four
sporozoites and a patch of residual protoplasm. (From Wasielewski,
after Balbiani.)]

[Illustratio