672 GARY N. CALKINS 



out of sight. During this middle period of division the organ- 

 ism remains quiescent or moves but sHghtly, but as the end 

 stages come on it moves more freely and with greater rapidity, 

 both undulating membranes in active motion. Not only are 

 new membranelles formed in the posterior half but the old 

 membranelles of the anterior portion are also replaced by new 

 ones (fig. 6). The new membranelles appear as minute cilia 

 within the old row and grow quickly, moving at first with a 

 spasmodic activity quite different from the regular wave-like 

 motion of the older membranelles. As growth of the new mem- 

 branelles progresses the regularity of motion increases until two 

 full sets of membranelles are in play. This condition lasts but 

 a few minutes, however, as the older set begins to fade away 

 by absorption until finally replaced by the active new set. 



None of the older observers was able to make out a micro- 

 nucleus in the ordinary vegetative stages, nor during division, 

 but Btitschli observed two 'encapsulated' smaller nuclei during 

 the process of conjugation of B. lateritia which he regarded as 

 micronuclei. Balbiani ('60) also is said to have observed the 

 micronucleus in B. lateritia. Btitschli remarks on the diffi- 

 culty experienced by himself in demonstrating the micro - 

 nucleus and states that he was unable to identify it in the ordi- 

 nary vegetative forms, although he did not doubt its presence 

 in the ordinary cells, since he found it in multiple number in 

 conjugating specimens. 



The difficulties in demonstrating the presence of micronuclei 

 have in no wise decreased with the lapse of time since the obser- 

 vations of these gifted pioneers. Different methods, however, 

 and especially the use of sections, have enabled us to trace 

 these elusive bodies and to get some light on their history, al- 

 though it must be admitted that this history does not corre- 

 spond with that of any other known ciliate. They are intimately 

 associated with the macronucleus, and in resting phases of the 

 cell I have failed to find positive evidence of their presence. 

 During division, however, they appear as extremely minute, 

 deeply-staining bodies which apparently divide within the 

 nuclear membrane of the macronucleus (fig. 8). In total 



