126 J. J. LISTER. 



are also mentioned, which it is suggested may have been of the 

 nature of nuclei. 



Polystomella striatopiinctata, F. and M. Schulze's account 

 ((42), p. 18) of the nucleus of this species is the most complete 

 that has yet been given. It is described as a round body 

 having, in some cases, a diameter of 56 fj,, and surrounded by a 

 conspicuous membrane of considerable thickness. Highly re- 

 fracting nucleoli, lying in a clear and apparently fluid substance, 

 occupy the interior. In quite young specimens only one 

 nucleolus was found, but they may number as many as twenty 

 or more in advanced specimens. The nucleus occupies a posi- 

 tion in the middle third of the series of chambers, and often 

 lies partly in one of the narrow canals connecting the chambers, 

 being preserved in its progress from one chamber to another. 

 ScHULZE points out that the position of the nucleus is thus 

 dependent on the total number of chambers. 



Sometimes two,, and in one instance three, nuclei were 

 present, and in this case they occupied adjoining, ,or at least 

 neighbouring, chambers. 



Verworn ((46), p. 462) has made a very interesting obser- 

 vation on the nucleus of Polystomella. On breaking au indi- 

 dividual into pieces, he . finds that,, while many of the larger 

 pieces remain, alive, as shown by their extended pseudopodia, 

 only the fragment containing the nucleus possesses the power 

 of secreting fresh material to repair the broken shell. 



HoFER ((19), p. 149) objected to Verworn's experiment on 

 the ground that, in the Polystomellas investigated by him, by 

 far the greater number of specimens had many nuclei, so that 

 when a shell is broken it would be improbable that any conr 

 siderable fragment would be free from nuclei. I can only say 

 that Hofer's experience is very diiferent from mine, as it will 

 appear later. 



At first sight much of the evidence here collected appears 

 contradictory. 



It will be shown below that in Polystomella the megalo- 

 spheric form has a single nucleus during the greater part of its 



