1922] KofoidSwezy: Mitosis and Fission in Giardia cnterica 219 



The cyst wall is a hyaline, homogeneous structure of uniform thick- 

 ness throughout, less than 0.5/^ in thickness. Its formation evidently 

 begins as a secretion about the as yet uncontracted 1-zooid body. In 

 figure 13, plate 24, an individual is shown in which the cyst is in the 

 process of formation, while the tail is not, as yet, fully retracted 

 within the ellipsoidal mass characteristic of the encysted phase, and 

 the cyst wall shows two separated layers posteriorly, the inner one of 

 which has the contour of the free stage. 



The following structures only can be identified as taking part at 

 some stage at least in multiple mitosis : the nuclei, centrosomes, 

 blepharoplasts. parabasals, axostyles, intracytoplasmic parts of the 

 posterolateral and the anterolateral flagella including the anterior 

 node. Free flagella and the peristomal fiber disappear and the rhizo- 

 plasts are found with difficulty, if at all. 



The sequence of events within the cyst is as follows : Encystment 

 of the 2-nucleate, 1-zooid flagellate occurs within the bowel, presum- 

 ably at or near the locus of infection. Since cysts are discharged in 

 fresh stools in various stages of multiple fission, it is certain that suc- 

 cessive mitoses proceed as the cysts are in transit through the bowel, 

 but that the rate and stage attained vary among the cysts. Many of 

 them are in the 4-nueleate, 2-zooid and 8-nucleate, 4-zooid stages, few 

 are discharged in the 2-nucleate, 1-zooid, and 16-nucleate, 8-zooid stages. 



The individuality of the zooids formed by mitosis in the period 

 prior to plasmotomy, M'hieh is so marked in the struggling zooids of 

 the 8-eelled somatellas of Trichomonas (Kofoid and Swezy, 1915), 

 is manifested in Giardia at the close of the first mitosis when the two 

 pairs of nuclei and their attendant neuromotor systems may, in some 

 cysts, shift to opposite poles of the cyst (pi. 25, fig. 21 ; pi. 26, figs. 24, 

 30). That they do not always attain or retain this position is indicated 

 by the occurrence of 4- and 8-nucleate cysts with the nuclei all at one 

 end (pi. 26, figs. 23, 28). This variability in position suggests some 

 mobility and functional and structural individuality of the constituent 

 zooids of the cysts of the as yet common cytoplasmic mass. In other 

 words, the individuality of the binucleate zooids is maintained through- 

 out the life of the cyst and the common cytoplasm appears to be merely 

 the containing medium. The cyst of Giardia is not comparable with 

 a cleaving egg but rather with a multiple cysticercus or chain of 

 planarians. It is asexual reproduction of the Giardia zooid which 

 progresses in the cyst. 



Plasmotomy does not often occur within the cysts, in so far as we 

 have observed them, though seen in G. mtiris, and duplications of the 



