202 FAMILY: AM(EBID.E 



The cyst of E. histolytica, when seen in fresh material, has a greenish 

 refractile appearance. Owing to its refractiveness, which is much more 

 marked than that of the cysts of E. coli, it is sometimes very difficult to 

 distinguish the nuclei, though the chromatoid bodies may be easily seen. 

 In iodine solution, however, all the contents can be clearly distinguished 

 (Plate II., 5-IO, p. 250). 



The cysts of E. histolytica vary in diameter from 5 to 20 microns 

 according to the particular race, but all the cysts of any one race are not 

 of the same size. Thus, in six cases studied by the writer and O'Connor 

 (1917) the diameter of the cysts varied as follows: 7 to 9 microns, 7 to 11 

 microns, 10 to 13 microns, 10 to 14 microns, 11 to 15 microns, 12 to 18 

 microns. The cysts remained constant in their average size during the 

 observation, which in some cases extended over several months, so that 

 it would appear that true races are represented (Fig. 10). Some observers, 

 however, believe that the small cysts belong to a distinct species of 

 amoeba. Prowazek (1912 a) gave the name E. hartmanni to these forms, 

 Kuenen and Swellengrebel (1917) the name E. tenuis, and Brug (1917) 

 the name E. minutissima. Though the writer has repeatedly observed 

 the appearance of cysts in the stools of cases in which the acute symptoms 

 of amoebic dysentery were subsiding, these have always been of the average 

 size, or larger than this. In no case has he seen the small cysts appear 

 under these circumstances. It cannot be regarded as finally established 

 that the races of E. histolytica which produce the small cysts are able 

 to give rise to amoebic dysentery. Drbohlav (19256) has cultivated a 

 small race. In the cultures the amoebae resembled the typical 

 E. histolytica. They did not, however, ingest red blood-corpuscles, and 

 though producing infection in kittens, failed to give rise to dysentery and 

 ulceration of the large intestine. The precystic amoebae as seen in faeces 

 correspond in size with the cysts, so that they are smallest in those 

 races which produce the smallest cysts. There are no data, however, 

 to show whether a corresponding variation in average size of the 

 tissue-invading forms occurs. Shimura (1918) described a race of 

 E. histolytica with small cysts as a non-pathogenic race, but it has to be 

 remembered that the majority of carriers who show no symptoms are 

 passing cysts of the average size. If carriers alone were examined, the 

 average-sized cysts might with equal justification be regarded as belonging 

 to non-pathogenic races. In the case of the smaller-sized cysts, diagnosis 

 from living specimens may be very difficult unless the characteristic 

 rod-like chromatoid bodies are present. In iodine the details are much 

 clearer, but it is often necessary to prepare stained films before making 

 a final diagnosis. 



The cysts passed from the body may contain one, two, or four nuclei. 



