58 THE AMOEBAE LIVING IN MAN 



Ihrough a small aperture. The nuclei of these amoebae were not inves- 

 tigated, however, and they state that " from our living (sic) observations, 

 we could not tell whether the amoebae which excysted had always four 

 nuclei." Nevertheless, the authors "are strongly of the opinion that this 

 is the normal method of excystation." The evidence for such strong 

 views is not indicated. The excysted amoebae are stated to have thrown 

 out pseudopodia, but they underwent no further development, and 

 finally died. 



Shortly after the publication of these experiments, Dr. A. C. Stevenson 

 and I attempted to repeat them. Although we carefully followed the 

 methods described, we never succeeded in causing the cysts to undergo 

 any development. After a number of failures, we came to the con- 

 clusion that E. histolytica will not usually excyst in liquor pancreaticns. 

 We thought it highly probable that among the cysts which Penfold, 

 Woodcock, and Drew employed, there were some which had only 

 recently been formed ; and that the walls of these might occasionally 

 be digested by* the pancreatic fluid, and so liberate newly-encysted 

 organisms still possessing some powers of movement. We could find 

 no evidence to prove that the authors had ever caused a mature 

 4-nucleate cyst to hatch — in spite of their firm conviction. I still think 

 that this is a probable explanation of their observations. 



More recently Cutler (1919) has recorded similar experiments. He 

 states that " all the excysted amoebae were uninucleate," and agrees with 

 the explanation just advanced. But he claims to have obtained more 

 satisfactory results by treating the cysts with liquor peptiais " tor a short 

 time," followed by liquor pancreaticns — a method which apparently failed 

 with Penfold, Woodcock, and Drew. By this method, the majority of 

 excysted amoebae are stated to be 4-nucleate, the author having been 

 " able to stain them with methyl green during their emergence." No 

 further development of these excysted amoebae was observed. But 

 Cutler adds that though he has " not seen the intermediate stages there 

 is evidence that these quadrinucleate amoebae ultimately divide to form 

 four small amoebulae." Except, however, for a figure of a small amoeba 

 — whose origin is not indicated — the evidence is withheld. 



It will thus be clear, I think, that the early stages in the development 

 of E. histolytica are still in doubt. Beyond a definite indication that the 

 cysts hatch in the small intestine, and some inconclusive evidence that 

 each liberates one 4-nucleate amoeba rather than four small uninucleate 

 organisms, there is little to go upon. It is probable that the 4-nucleate 

 amoebae divide into uninucleate individuals — if they really emerge in 

 this condition : but whether the new individuals, however formed, are 

 gametes which conjugate in pairs — as I suggested in E.rauarum (Dobell, 

 1909), and as Mercier (1910) claims to have shown in £. blattae — is still 

 a matter for speculation. 



Are there Races of E. histolytica differing in Pathogenicity f — It has 

 frequently been suggested that there are races or strains of E. histolytica 

 which differ in virulence or pathogenicity. This view is generally put 

 forward owing to a misconception of the carrier condition, or through 

 false analogies drawn between E. histolytica and the Bacteria. It merits 

 some consideration here, however, as it involves several important points 

 in the life-history of the parasite. 



In the first place, it may be pointed out that the existence of patho- 

 genic strains — causing amoebic dysentery, liver abscess, etc. — and non- 



