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 trophozoite 

 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 schizont 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 protecting envelope or oocyst. After 

 the oocyst is formed the parasite may pass from the host to 

 the exterior or remain for some time longer within it. The 

 nucleus of the zygote within the oocyst now divides into 

 four, around which the protoplasm aggregates itself to form 

 the spores. There are thus four spores within a cyst. 

 Each spore divides, forming two sporozoites, which on the 

 arrival of the oocyst in the gut of a fresh host are liberated, 



