114 BIOLOGY OF THE PROTOZOA 



their possible relations to one another or to any process of evolution. 

 The apparent object of the complex mechanism of a mitotic figure 

 is to ensure the exact bipartition of the hereditary complex repre- 

 sented by the chromosomes. These elements, and the chromatin of 

 which they are composed, are the most important, while the kinetic 

 elements with which they are associated in division, as agents in the 

 process, are of secondary importance. I shall consider first, there- 

 fore the chromatin of protozoan nuclei and its history during nuclear 

 division. 



(a) Chromatin and Chromosomes.— The conception of chromo- 

 somes, as they appear in Metazoa, is definite and consistent through- 

 out. They are formed at certain periods of cell activity (prophase 

 of division) by the aggregation of chromomeres into nuclear bodies 

 of definite form and size, and the number is constant for all somatic 

 and germ cells in the same species. Each chromosome is specific 

 and retains its individuality from generation to generation by 

 cell division. At the end of division it resolves itself into an aggre- 

 gate of chromomeres which, in some cases, are found to be confined 

 to a definite part of the nucleus (chromosomal vesicle) at the pro- 

 phase of the following division these same chromomeres re-collect 

 to form the chromosome which divides into equal parts hx longi- 

 tudinal division. The chromosomes, furthermore, are qualita- 

 tively different, no two of them being identical. At one meiotic 

 division, finally, the number of chromosomes is reduced to one-half 

 hy the separation of half of them from the other half, thus resulting 

 in two types of nuclei which are entirely difi'erent in chromosomal 

 make-up. 



An analysis of the literature dealing with the so-called chromo- 

 somes of Protozoa shows that there has been little or no consistent 

 use of the term. To many observers the word is used to describe 

 any chromatin which happens to be in the center of a division figure 

 and without regard to other conditions which limit and define 

 the chromosome as a definite thing. Adz.: A definite number in the 

 cell, longitudinal division, qualitative differences, reduction in 

 number at maturation, etc. It is true that in only a few cases among 

 the Metazoa has it been demonstrated that chromosomes have a 

 specific individuality combined with qualitative differences, but 

 the striking similarity in dividing chromosomes of all Metazoa and 

 the same complicated mechanism in all cases for their equal distri- 

 Inition to daughter cells, give a basis upon which the generalization 

 rests. We have no basis, however, for extending the generaliza- 

 tion to Protozoa, for here we have absolutely no evidence of quali- 

 tative dift'erences and no evidence of individuality. In some cases 

 we have e\'idence that structures in the center of a division figure 

 are formed l)y the fusion of chromomeres, and some evidence that 

 such structures divide longitudinally. These two conditions, which 



