No. 466.] STUDIES ON PLANT CELL. — VIII. 729 



have a distinct organization which holds throughout the hfe his- 

 tory. The form of the chromosomes certainly does change with 

 different periods of the cell's history especially within the rest- 

 ing nucleus and yet the centers of chromosome activity may 

 always be present to organize the chromatin into a new set of 

 elements for the next mitosis. It is perhaps difficult to believe 

 that the chromatin granules (chromomeres) find their way back 

 to the same chromosome with the prophase of each mitosis but 

 the existence of chromosome centers may be readily conceived 

 within the resting nucleus which would hold the number of chro- 

 mosomes true to the cell's history. 



With respect to the nucleolus there is abundant evidence that 

 the structure is not a permanent organ of the cell. When con- 

 taining chromatin, the nucleolus is found in its characteristic 

 globular state only during the resting condition of the nucleus. 

 Its chromatic substance passes into the chromosomes at pro- 

 phase of mitosis and the nucleolus generally disappears before 

 metaphase. Or if any substance is left after the chromosomes 

 are formed the remaining structure either gradually dissolves or 

 is thrust forth bodily into the cytoplasm surrounding the mitotic 

 figure where it disappears sooner or later. The nucleolus in 

 higher types of mitosis never divides to pass on with the chro- 

 mosomes to the daughter nuclei, but such a history is reported 

 in the yeast cell. If the nucleolus has any function in heredity, 

 as has been claimed (Dixon, '99), such function must relate to 

 the chromosomes which contribute to its substance or derive 

 material from it. Besides the nucleoli which are composed 

 wholly or largely of chromatin, there are also those which seem 

 to have httle if any relation to the chromosomes. Such are 

 well known in the spore mother-cells of higher plants and no 

 investigator has been able to connect these with the formation 

 of chromosomes as Wager (: 04) has been able to do in the root 

 tip. It was upon nucleoli of this class that Strasburger founded 

 his theory that the structure was a mass of reserve material 

 utilized by the kinoplasra during mitosis in the process of spindle 

 formation. Such nucleoli generally fade away during the pro- 

 phase of mitosis and either entirely disappear or the remaining 

 substance is thrust out into the cytoplasm where it may some- 



