No. 466.] STUDIES ON PLANT CELL. VIII. 727 



and chromosome history in relation to problems of development, 

 heredity, hybridization, and variation is clearly understood, and 

 these subjects have already been treated in Section V, " Cell 

 Activities at Critical Periods of Ontogeny in Plants." Also 

 some recent papers on the nucleolus of which Wager's (: 04) is 

 the most comprehensive, have brought this structure into very 

 close relation with the chromatin content of the nucleus, and the 

 nucleolus must now be considered in any treatment of the chro- 

 mosomes. The problems hinge on what is termed the individu- 

 ality of the chromosome, which is the question whether or not 

 the chromosome is a structural entity maintaining its independ- 

 ence completely through each and all of the cell divisions in a 

 life history. There is also involved the view that the chromo- 

 somes have come down from a line of ancestral structures, 

 reproducing by fission in every mitosis throughout the history 

 of the race. 



There are two extremes in the views on this exceedingly 

 interesting conception and also an intermediate position. The 

 one extreme has recently been set forth by Boveri (: 04) in a 

 very clear statement. This view regards the chromosomes as 

 structural entities, possibly elementary organisms, which main- 

 tain an organic individuality and independent existence in the 

 cell. They are further regarded as in their typical form when 

 present as rods or short filaments during mitosis. Their be- 

 havior in the resting nucleus is one of great metabolic activity 

 which affects their morphology for the time being. 



Those who are inclined to doubt the individuality of the chro- 

 mosomes and to hold off from a full acceptance of the theory, 

 base their attitude on the extreme difficulty or perhaps impossi- 

 bility of following the chromosomes as entities through the rest- 

 ing nucleus from one mitosis to another. These difficulties are 

 well known to those who have studied chromosomes even in 

 nuclei which are most favorable for the investigation of their 

 morphology. The chromosomes which enter the daughter 

 nuclei from a mitosis generally lose their form and the chroma- 

 tin becomes so distributed on a linin network or in a nucleolar 

 structure that the outlines of the original structures become 

 quite lost. Mottier (: 03) in his recent studies on the spore 



