40 THE ANIMAL PARASITES OF MAN 



is disputed by some authors. The round cysts, which may measure 

 I2//, to 15 //, in diameter, contain four nuclei, together with darkly 

 staining masses of various shapes, the so-called " chromidial blocks" 

 (fig. 9, /). The cyst-wall of E. histolytica (tetragena} is thinner than 

 that of E. coli, and the diameter of the cyst is rather less. E. histolytica 

 has not yet been cultivated. 



Infection in man occurs by way of the mouth by the ingestion 

 of cysts. A patient showing acute symptoms of dysentery is not 

 usually infective, for he is merely harbouring the large trophozoites, 

 which, by experiment, have been shown not to be infective to animals 

 (kittens) when administered by the mouth. The stools of recovered 

 patients may still contain cysts, and they may thus act as cyst-carriers 

 or reservoirs of disease by infecting water and soil. The stools of such 

 cyst- carriers are often solid, and so cysts of E. histolvtica (tetragena) 

 are easily overlooked. Mathis (I9I3) 1 points out that healthy carriers 

 of E. histolytica may be found ; 8 per cent, of the natives of Tonkin 

 examined by him were healthy carriers of cysts. 



In return cases, or prolonged untreated cases of entamcebic 

 dysentery, a generation of smaller trophozoites is associated with, or 

 replaces the larger ones. In stools they are frequently refractile and 

 consequently stain slowly inira vitam. These trophozoites are the 

 " smaller, senile, or pre-cyst generation " of Darling. This pre-cyst 

 generation is characterized by the presence of blocks of crystalloidal 

 substance in the cytoplasm, and by the possession of a prominent, 

 densely stainable karyosome. Darling believes this generation to 

 be the same as that described by Elmassian as Entainceba. minuta? 

 Walker, 3 Darling, 4 Wenyon 5 and others believe that Entainceba 

 histolytica, which was only seen by Schaudinn in a single case, that of 

 a Chinaman, is really E. tetragena. Darling states that if the published 

 illustrations of E. histolytica and of E. tetragena are collected from the 

 literature and compared, it will be seen that the writers have been 

 calling E. histolytica the large trophozoites seen in dysenteric stools. 

 These large trophozoites frequently display no karyosome, but they 

 can be demonstrated as E. tetragena by animal inoculation, or by the 

 history of the case. On the other hand, the illustrations of E. tetragena 

 show that the authors have been dealing with the small generation or 

 reduced forms (" E. minuta. "), which are the direct descendants of the 

 large trophozoites. If kittens are inoculated rectally with dysenteric 

 material containing large trophozoites, the strain may be carried in 

 successive kittens for four to six transfers. If, on the other hand, 



1 Bull. Soc. Med. et Chirurg. Indo-Chine, iv, p. 474. 



- Centralbl.f. Balder., Orig., lii, p. 335. 3 Philip. Journ. Sc. (1911), B, vi, p. 259. 



4 Annals Trap. Med. and Parasitol. (1913), vii, p. 321. 



5 Brit. Med. Journ., Nov. 15, 1913, p. 1287, and fourn. Lond. School Trap. Med., ii, p. 27. 



