200 FAMILY: AMCEBID^E 



accurately studied by Huber (1903), though the cysts had previously been 

 seen and figured by Quincke and Roos (1893), and Rods (1894), They 

 were again seen by Viereck (1907), and by Hartmann and Prowazek (1907), 

 who regarded them as belonging to distinct species of amoebae, which 

 were named E. africana and E. tetragena respectively. This supposed 

 difference, however, was the outcome of Schaudinn's erroneous account 

 of the development of E. histolytica, which was almost entirely based on 

 the appearances seen in degenerating amoebse. There can be no doubt that 

 E. africana, E. tetrageyia, and E. ttmiuta are merely forms of E. histolytica. 

 The small amceba described by Prowazek (1912a) as E. hartmanni, and 

 by Kuenen and Swellengrebel (1917) as E. tenuis, is undoubtedly a small 

 race of E. histolytica, producing cysts 6 to 8 microns in diameter. 



3. Cyst. — The cyst which is formed round a precystic amoeba seems 

 to be composed at first of a soft material which quickly shrinks and 

 hardens to a resistant, colourless, smooth, transparent capsule. It is 

 completely filled by the cytoplasm of the amoeba (Fig. 96). The cyst 

 wall is about 0*5 micron in thickness, its inner and outer margin being 

 visible in optical section. When first formed, it encloses the amoeba 

 and the structures it contains. Thus, the newly-formed cyst contains 

 the cytoplasm and nucleus, and also the vacuole and chromatoid bodies 

 if these happened to be present in the amoeba. The cysts are generally 

 spherical, but they may be elongated or even dumb-bell-shaped. Within 

 the cyst the single nucleus divides to form two nuclei, and these divide 

 again, so that in the mature cyst four nuclei are present (Fig. 57). Very 

 frequently the four nuclei are arranged in pairs at opposite sides of the cyst. 

 On Very rare occasions eight nuclei may be found. According to Dobell 

 (1919) the vacuole, if present, gradually disappears as the cyst develops. 

 It appears as if the glycogen of the vacuole is used up during the nuclear 

 divisions. The chromatoid bodies are similarly absorbed while the cyst 

 is waiting outside the body to be ingested by a new host. The chromatoid 

 bodies usually have the form of rods with rounded ends, and very com- 

 monly one, two, or three are present in the cyst. They vary in length 

 from 5 to 10 microns, but longer or shorter forms may occur. Sometimes 

 they ^e of a different shape, and may be more rounded or irregular in 

 outline. On other occasions they are filamentous structures, or a large 

 number of small, irregularly-shaped bodies may be present. When seen 

 in the living cyst they appear as homogeneous structures which have 

 a refractive index higher than that of the rest of the cyst. On this 

 account they are readily distinguished. The chromatoid bodies are of 

 great diagnostic value, for they occur much more rarely in the cysts of 

 E. coli, in which case the eight nuclei characteristic of the mature cysts 

 of this amoeba will be noted. 



