No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 527 



Synchytniim) ; Reinke studied the mitosis of the spleen cells 

 of the mouse, and found that the single parent-nucleolus divides 

 into three or four pieces, while at the end of the mitosis each 

 daughter-nucleus contains a single nucleolus. In the mitoses 

 of the ovogonia of Lifietis and Polydora my own observations 

 show that the nucleolus persists in the nucleus, and each 

 daughter-nucleus contains one nucleolus, so that it is very 

 probable that in these cases the parent-nucleolus divides into 

 two, and each daughter-nucleus thereby receives a half of it ; 

 but these mitoses were so small that I was unable to decide 

 this point definitely. Rosen (-95) finds nucleolar division in 

 root cells of Phaseohis ; J. Wagner ('96a) describes a similar 

 division of a " nucleolus " in spermatocytes of Arachnids, though 

 this case, like that described by Henking ('90), probably repre- 

 sents a chromatin nucleolus. This persistence of the nucleolus 

 in the nucleus during mitosis must be considered atypical. 



{b) The nucleolus disappears during mitosis. — This is the 

 most usual mode of behavior of the nucleolus during mitosis. 

 The nucleolus either gradually diminishes in size, and so finally 

 vanishes, or else it first fragments into a number of smaller 

 pieces, and then these disappear. The only cell which I had 

 for the study of this phenomenon was the ovum of Piscicola 

 during the formation of the first pole spindle. When this 

 spindle is complete no trace of nucleolar substance is to be 

 seen anywhere in the cell. In stages immediately antecedent 

 to that of the spindle, numerous minute granules, as well as a 

 smaller number of larger globules, are dispersed through the 

 nuclear sap; all these stain with eosin, and I regard them as 

 particles of nucleolar substance which had become separated 

 from the nucleolus. Thus a dissolution of the nucleolar sub- 

 stance commences before the nuclear membrane has disap- 

 peared, and after this membrane has vanished it is probable 

 that all the nucleolar substance must be dissolved by the action 

 of the cytoplasm, or at least become dispersed through the 

 latter, so that no remnant of it is to be found in the region of 

 the spindle or of the chromosomes. During the process of 

 dissolution of the nucleolar substance in the nuclear sap the 

 chromatin elements stain red (with eosin), and this fact may be 



