COCCIDIUM. 105 
a bow and straightening out again. When about to enter 
an epithelial cell it presses the anterior end through the cell 
wall and wriggles its way in. Once within the cell in which 
development is to proceed, its movements gradually cease, 
but it may pass through several cells before coming to rest. 
Within the host cell the coccidium—now in the ¢rophozotte 
stage—becomes oval in form, and in about twenty-four hours 
has reached full size and has exhausted the host cell 
contents. This is the completion of the trophozoite period, 
and the parasite now enters the schizon¢t stage, where its 
nucleus divides into a number of daughter nuclei. These 
arrange themselves around the periphery of the cell, whilst 
the protoplasm breaks up to form along with them bodies 
of a shape similar to the sporozoites. There are important 
structural differences, however, apart from the difference in 
origin. The parasites, now known as merozoites, rupture 
the host cell, move in the gut cavity after the manner of the 
sporozoites, enter fresh epithelial cells, and repeat the fore- 
going cycle until ultimately the greater part of the gut 
epithelium is destroyed. In about five days, however, 
owing perhaps to the failing capacity of the host to nourish, 
the limit of asexual reproductivity is reached, and the 
parasite now enters upon a spore-forming stage. Certain 
merozoites grow more slowly than the others, and instead ot 
becoming schizonts give rise to elements of two types, viz. 
microgametes, slender cells bearing a flagellum at each end, 
which are male, and macrogametes, larger bean-shaped cells, 
which are female. The latter after maturation free them- 
selves from the host cell, and in the cavity of the gut are 
fertilised by a male element. After fertilisation, a trans- 
parent membrane forms around the zygote (fertilised cell). 
This membrane in the first instance serves to exclude all 
microgametes after the first, and later, becoming very tough 
and resistant, forms a pretecting envelope or odcys¢. After 
the odcyst is formed the parasite may pass from the host to 
the exterior or remain for some time longer within it. The 
nucleus cf the zygote within the odcyst now divides into 
four, around which the protoplasm aggregates itself to form 
the speres. There are thus four spores within a cyst. 
Each spore divides, forming two sporozoites, which on the 
arrival of the odcyst in the gut of a fresh host are liberated, _ 
