44 TJmversity of California Publications in Zoology [Vol. 16 



Multiple Mitosis in Cysts 



Multiple mitosis also occurs in encysted individuals and is pre- 

 sumably followed by multiple fission. All stages in the formation of 

 a 16-nucleate somatella, including the 2-, 4-, 8-, and 16-nueleate cyst 

 have been found (pi. 7, figs. 35-44). Four chromosomes occur here as 

 elsewhere at mitosis though they are somewhat more fused in paii-s. 

 The daughter nuclei retain only approximately a bilateral grouping 

 and ultimately spread through the cytoplasm from end to end (pi. 7, 

 fig. 44). The axostyles do not exceed two in any of our 16-nucleate 

 (8-zooid) cysts. The chromatic margins of the peristome and the 

 parabasals disappear, diffuse deeply staining masses appear in their 

 places, and the two halos disappear. The posterior peristome forms 

 a deeply stained thick semicircle (pi. 7, fig. 38) attached to the pos- 

 terior ends of the intracytoplasmic part of the anterior flagella which 

 persists after the rest of the extensions of the neuromotor apparatus 

 has disintegrated, and finally breaks up into a thin granular line 

 (pi. 7, fig. 42). The intracytoplasmic ends of the po.stero-laterals 

 persist for some time as dark lines (pi. 7, fig. 41). 



The cyst wall is thin, hyaline, double-contoured, and is generally 

 closely applied to the body, though in some cases (pi. 7, figs. 38. 39) 

 pofisibly due to shrinkage, there is an intervening space. No empty 

 cysts have been found, so that presumably the final phase of multiple 

 fission, the detachment of eight fully equipped .small individuals .suc- 

 cessively from the escaped 16-nucleate Plasmodium, or their escape 

 singly from the cyst, takes place outside of the host in which they are 

 produced, or possibly in a new host. 



Encystment 

 Three types of cj^sts (which may be reducible to two) occur. (1) 

 Cysts in which multiple fission is in progress or completed (pi. 7, 

 figs. 35-37, 41-44). (2) So-called copulation cysts containing two 

 detached individuals (pi. 8, fig. 61). (3) Possible maturation stages 

 in these "copulation" cysts. Such a cyst in G. microti (pi. 8, figs. 

 62-63) shows evidence of the two individuals, apparently back to back, 

 and with the anterior ends at oposite poles as in the earlier (gameto- 

 cyte?) stage (fig. 61) and in certain free .stages (pi. 8, fig. 55). It is 

 not certain that these are maturation stages rather than multiple fission 

 of two individuals in the stage of advanced plasmotomy (figs. 31-34) 

 which have swung into a back-to-back position and encysted (fig. 61). 



