ENTAMOEBA HISTOLYTICA 65 



(1916) describe a stage which they beheve to be "a female gamete"; but 

 they observed no conjugation or other sexual process, and no reasons 

 are given for their peculiar interpretation. Mathis and Mercier (1916 d, 

 1917 b) have asserted that there are two different kinds of cysts in this 

 species — " microcysts," measuring 12-5//, in diameter, and "macrocysts" 

 of 14//.: and they suppose that these, when ingested, liberate micro- 

 gametes and macrogametes respectively, which conjugate with one 

 another. It has been sliown (Dobell and Jepps (1918) ; Malins Smith 

 (1918)) that this dimorphism of the cysts is not really observable; and 

 consequently the rest of their hypothesis is deprived of its chief evidence. 



It is not impossible, however, that conjugation does occur at the 

 stage in the life-cycle suggested by Mathis and Mercier — namely 

 between the amoebae liberated from the cysts in the small intestine at 

 the beginning of their life-history. Nevertheless, this has yet to be 

 observed. 1 originally suggested (1909) that such a development might 

 occur in the very closely related species E. rananiin, and Mercier (1910) 

 has since described it in E. blaltae. But his observations have not yet 

 been confirmed, and at present there is no proof that conjugation occurs 

 in any other parasitic amoeba. 



Other Interpretatioiisof the Life-history. — It is now generally recognized 

 that the life-history of E. histolytica follows the course here described. 

 Since the appearance of the works of Walker (191 1, 1913), any other 

 interpretation of the development of this parasite has, I think, become 

 untenable. My own experience has, at all events, convinced me over 

 and over again that his conception of the life-cycle as a whole is the only 

 correct one. It agrees entirely, moreover, with that of £. rauarinu, which 

 I had worked out previously. In both species there is a large amoeboid 

 form which represents tiie ordinary vegetative and reproductive stage. 

 In certain circmiistances this form produces — probably by simple 

 division — a smaller precystic form which does not feed in the usual 

 way, but gets rid of any food it may happen to contain, and then 

 encysts. Within the cyst a nuclear multiplication occurs, by successive 

 divisions, until four nuclei are formed. The cyst is then mature, and 

 ready to infect a fresh host. The whole life-history is extremely simple, 

 and is only complicated, in E. histolytica, by the occasional wandering of 

 the parasite from its normal habitat in the gut wall into other tissues. 

 The chief difference between E. ranariiin and E. histolytica is in habit — 

 the one being a simple commensal, the other a true parasite, incapable of 

 nourishing itself upon anything but living tissue. 



Several other views have been advanced concerning the life-history of 

 E. histolytica. Schaudinn's (1903) description is now universally admitted 

 to have been incorrect, and merits no further discussion. Kuenen and 

 Swellengrebel (1913, 1914), among more recent workers, hold peculiar 

 views concerning the life-history of E. histolytica. They believe (2Hde 

 Kuenen and Swellengrebel, 19 13) that the parasite, which they call — 

 without any justification — E. tetragena, occurs in three "phases": a 

 " histolytica phase," in the tissues in dysentery, liver abscess, etc. ; a 

 " ininuta phase," living as a harmless commensal like£. coli in the lumen 

 of the gut ; and a " tetragena phase " in which the parasite appears in the 

 encysted form. They consider that the " minuta phase," which is 

 saprozoic, can "reproduce" the '^histolytica phase," which is parasitic 

 in the tissues, and vice versa. They do not regard the " iiiiniita " forms as 

 precystic amoebae ; though — if I understand them correctly — they believe 



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