1922] Kofoid^Smezy : Mitosis and Fission in Giardia cnterica 221 



Dedifferentiation in Encystment 



During the encysted period those parts of the organism functioning 

 in locomotion and attachment dedifferentiate, namely, the free ends 

 of the four pairs of flagella, the sucker-shaped eytostome, and per- 

 istomal fiber, all of which disappear promptly when the body rounds 

 up within the cyst wall, except the posterior peristomal fiber, which 

 lingers for a time. A similar dedifferentiation does not occur in the 

 mitosis of the free stage. Even the newly formed intraeytoplasmic 

 parts of the anterolateral flagella stain feebly and lag in formation 

 in the cyst. Similar parts of the posterolaterals, however, stain heavily 

 and are more evident. A similar phenomenon appears in somatellas 

 of Trichomonas, in which the homologue of the posterolateral of 

 Giardia, the marginal fiber of the undulating membrane, is deeply 

 stained during multiple fission. 



The posterior peristomal fiber shifts in location with the disappear- 

 ance of the eytostome and the anterior peristomal fiber. Its median end 

 moves posteriorly and the two sides form a V-shaped sti'ucture (pi. 25, 

 fig. 18) which gradually fades out. In this condition the peristomal 

 fiber tends towards the more elongated condition seen in Tricliomonas 

 and Chilomastix. The connection of this V-shaped structure with the 

 anterior part of the peristomal fiber persists for a time (pi. 25, fig. 22), 

 but the former soon disappears and only the latter can be found. It 

 is not to be confused with persistent intraeytoplasmic parts of the 

 posterolaterals or the parabasals. 



On the other hand, the axostyles and parabasals are enlarged and 

 stain more deeply in the cysts than in the free stages and their 

 duplication proceeds in step with the mitoses. The axostyles are 

 important organelles in the separation of the zooids at plasmotomy of 

 the somatella, and the parabasals appear to be concerned with meta- 

 bolism, perhaps in a fashion analogous to that of the chromatoidal 

 bodies in amoebic cysts in stools, and are therefore not dedifferentiated 

 as are the functionless, free flagella. 



