3i6 



PLASMODROMATA AND SARCODINA 



5 . Chromatin has left the kary osome margin and passed over the 

 linin network towards the nuclear membrane (Fig. 57, e). 



6. The centriole swells and re-forms the kary osome, and so the 

 cycle begins again or the amoeba divides (Fig. 58, b), 



Life-History.— L . histolytica may reproduce by binary fission 

 during the vegetative stage, or it may encyst. 



The first is the method by which it increases its numbers in a given 

 host, and the second is for the purpose of passing from one host to 

 another. 



There is no sure evidence of schizogony. 



Simple Division.' — ^The centriole divides into two parts, each of 

 which travels to opposite sides of the karyosome and are connected 

 by a thread, the centrodesmose. The further stages require study. 



Encystment — Precystic Phase. — All the amoebae present in a given 

 case reduce in size (L. minuta), but their numbers are augmented. 

 The amoebae become sluggish and have a clear ectoplasm, but the 

 nucleus is not easy to see, and chromidia may or may not be present 

 in the cytoplasm. The nucleus is spherical, with a thick regular 

 membrane and with chromatin granules near the periphery. The 

 karyosome shows no centriole. 



Fig. 58. — Loeschia histolytica Schaudinn: Old Trophozoite Phase showing 

 THE First and Last Stages of the Cyclical Nuclear Changes. 

 (x 1,950 Diameters.) (After James.) 



Cystic Phase. — Different strains produce c^^sts of different sizes — 

 e.g., small cysts 7-10, medium 9-12, large 12-18 microns in diameter. 

 Chromidia are formed in the cytoplasm, and may be derived from 

 the nucleus, and may function as food material. The cytoplasm 

 becomes granular and the nucleus swollen and elongated. An 

 intranuclear spindle is formed between two centrioles situate at 

 each end. The chromatin forms rows of granules extending from 

 pole to pole. The nucleus divides and the daughter nuclei enter the 

 resting stage, in which they remain for a long time. Each of these 

 subsequently again divide, giving rise to the typical quadrinucleate 

 cysts. Tenfold, Woodcock, and Drew have brought evidence to 

 show that, as a rule, the amoeba escapes undivided from the cyst. 

 No one has so far seen any flagellate forms or any gametes. 



Infection.— Darling in 191 3 showed that only the cysts were in- 

 fective, and that they are ingested with food, and must not have 

 been passed out of the original host longer than two or three days. 



Wenyon and O'Connor in 1916 have proved that, as in the case 

 of L. coli, house-flies can ingest the cysts, which are subsequently 

 passed on to human food, and so the infection carried from man to 

 man. 



