12 University of California Publications in Zoology [Vol. 20 



MULTIPLE FISSION 



Multiple fission in Streblomastix is a cyclic process occurring in 

 many individuals in a single host at one time. A few only may be 

 found in this stage, or, in some instances, at least, the majority of 

 individuals may be in the multinucleate phase. 



This condition is preceded by the growth of the schizont from the 

 small size resulting from multiple (pi. 2, fig. 13) or binary (fig. 17) 

 fission to a much larger or even giant stage (pi. 2, fig. 13), which 

 may be as much as twenty-six times the length of the smallest schizont. 

 The body may have as much as sixty times the mass of the smallest 

 stages. The nucleus, however, does not increase proportionately, 

 remaining, in fact, at least in many instances, almost unchanged 

 (pi. 2, fig. 13). 



At some period during this increase in size multiple fission sets 

 in. Not all trophozoites entering upon it attain the maximum size 

 as will be seen on a comparison of figui'es 15 and 16 on plate 2. 

 It is possible that figure 16 represents only a detached section of a 

 larger plasmodium which is fragmenting, or it may be a small tropho- 

 zoite in the initial stages of multiple fission. 



Contrary to the behavior of the blepharoplast-flagella complex 

 in binary fission where it leads in division, preceding the nucleus, 

 we find that in multiple fission nuclear division by transverse con- 

 striction is taking place prior to the division of the blepharoplast 

 (pi. 2, figs. 14, 15). The type of nuclear division is the same as in 

 binary fission. "We have not seen stages of multiplication of the ble- 

 pharoplast or of plasmotomy. 



The linear form persists during the period of multiple fission and 

 the nucleus becomes an elongated axial chromatin thread which 

 becomes attenuate locally and parts transversely (pi. 2, figs. 13-16). 

 The number of nuclear segments varies, probably in a 2—4-8 sequence, 

 although irregularities in this are apparent. The largest number 

 observed is eight. This accords with multiple fission in other poly- 

 mastigotes (Kofoid and Swezy, 1915; Kofoid and Christiansen, 1915). 



There is some evidence that this stage is contractile and that when 

 foreshortened the nuclei slip by one another. This is not a common 

 condition and probably does not represent a rounded-up condition 

 obligatory for multiple fission but rather a passing response to 

 stimulus resulting in contraction. 



