1922] Kofoid-Sivezy: Mitosis and Fission in Giardia rntrrica 217 



Chilomastix, involve not only the origin of reversed symmetry, but 

 the additional factor of the union in one individual, not of two indi- 

 viduals of Chilomastix, but of Chilomastix and its mirror image. The 

 fibrillar union of the neuromotor systems of the two into one integrated 

 system perfects their functional coordination in one organism. 



Since the bilateral Hexamitidae combine in one organism the 

 sinistrally and dextrally organized halves, we may infer (Kofoid and 

 Swezy, 1920) that the sinistral half might have arisen in evolution by 

 a reversal of symmetry at one pole of the nucleus at mito.sis, resulting 

 in one daughter organism being the mirror image of the other, a 

 relation permitting union of the two in a single bilateral organism. 

 That such a reversal is dependent upon a stereometric reversal of an 

 organ-forming substance is an inference inviting further investigation 

 on other than morphological lines. 



The Encysted Phase 



Gysts of Giardia enterica are abundant at irregular intervals in the 

 stools of infected persons. In the intervals between the periods of 

 abundance they decline in numbers or disappear entirely for periods 

 of several days to several weeks. The numbers in which they may 

 appear are large. Porter (1916) reports a case in whieli a stool of 

 950 cc. contained, on computation, 14,440,000,000 cysts or 15,200,000 

 per cubic centimeter. In a case of giardiasis under daily examination 

 by us in New Yorlj for forty-two days, cysts were found on haemo- 

 eytometer count on seventeen of the forty-two with a maximum record 

 of 3,925,000,000 Giardia in a stool of 365 grams, or 21,216,216 per gram. 



The cysts are found in all stages of multiple mitosis, that of the 

 single zooid similar to the free stage (pi. 24, fig. 13) up to the 12- and 

 presumably the 16-nucleate, 8-zooid stage (pi. 26, fig. 31). The single 

 zooid and the 8-zooid stages are, however, very rare in our material. 

 The period of encystment in the bowel is one of multiple fission, but 

 apparently not of plasmotomy. The presence of a considerable pro- 

 portion of 2-zooid cysts in the faeces suggests the possibility that some 

 cysts do not proceed beyond binary fission. On the completion of 

 plasmotomy .such cysts might give rise to so-called copulation cysts 

 with two individuals in end-to-end, back-to-back position. 



Multiple fission gives rise successively to 2-, 4-, and 8-zooid stages 

 with 4, 8, and 16 nuclei respectively, and attendant, but often lagging 

 neuromotor systems lacking the free flagella, with cytoplasm as yet 



