362 RESEARCH IN PROTOZOOLOGY 



tributed throughout the whole fihn, it is best to discard the speci- 

 men and get a new one. 



Identification of the species of parasite in thick films. The red 

 blood cells throughout most of the thick film are laked out, so that 

 they no longer serve as a guide in the determination of species. 

 On the other hand, the thick film often affords parasites in large 

 numbers and in a greater variety of stages of growth than one 

 would find in the ordinary examination of thin films. When 

 parasites are abundant enough it is well to examine the margin of 

 the thick film where the red cells are partially fixed by drying. 

 In taking films it is a good plan to make a small, thinner exten- 

 sion to the thick drop or to place a thin dab of blood beside it. 

 These thinner places become sufficiently fixed by drying and 

 may be stained along with the thick part. 



A few general directions may assist in the identification of 

 species in thick films : 



Benign tertian (P. vivax) : In most specimens these parasites ap- 

 pear in different stages of growth and one can find older schizonts 

 (Plasmodia) easily recognized by their larger size, irregular 

 form, abundant chromatin and pigment. In some thick films the 

 outline of the enlarged host cell persists, even at the center of the 

 preparation ; at the margin of the preparation the host cell often 

 remains intact and may exhibit Schiiffner's dots. Benign tertian 

 rings or younger parasites usually have larger chromatin dots and 

 more abundant cytoplasm than do estivo-autumnal rings. When 

 the cytoplasm occurs in ring form, its outline is less regular than 

 in estivo-autumnal. 



Quartan (P. malarias) is regarded by some authors as hardly 

 distinguishable from benign tertian in thick films. In most cases, 

 however, the smaller and more compact rings and schizonts, and 

 the most abundant pigment, serve to distinguish them. In sporulat- 

 ing forms the quartan has only eight spores while the benign 

 tertian has sixteen or more. 



Estivo-autumnal (P. falciparum) : Rings may vary greatly in 

 size, but are generally smaller than those of benign tertian. Chroma- 

 tin dots are smaller and more often two dots occur in one ring. 

 Where many small rings and no plasmodia are seen, it is usually 

 safe to identify the parasite as estivo-autumnal. Crescents, char- 

 acteristic of estivo-autumnal, are easily recognized when typical in 

 form. But they sometimes change their shape in slowly drying 



