00 BIOLOGY OF THE PROTOZOA 



Protozoa the difficulty of treating them in any general way has been 

 admitted by all students of cytology as well as by protozoologists. 



1 shall endeavor here to convey an idea of this diversity and at the 

 same time to describe some of the more frequent types of division 

 figure without confusing the issue still more by my own views as to 

 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. 



The conception of chromosomes, as they appear in Metazoa, is 

 definite and consistent throughout. 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 gen- 

 eration to generation by cell division. At the end of division it 

 resolves itself into an aggregate of chromomeres which, in some 

 cases, are found to be confined to a definite part of the nucleus 

 (chromosomal vesicle), at the prophase of the following division 

 these same chromomeres re-collect to form the chromosome which 

 divides into equal parts by longitudinal division. The chromo- 

 somes, furthermore, are qualitatively different, no two of them 

 being identical. During meiosis, finally, the number of chromo- 

 somes is reduced to one-half by the separation of half of them from 

 the other half, thus resulting in two types of nuclei which are quite 

 different 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, viz. : A definite number in the 

 cell, longitudinal division, qualitative differences, reduction in num- 

 ber 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- 

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

 rests. We have no basis, however, for extending the generalization 

 to Protozoa, for here we have absolutely no evidence of qualitative 

 differences and but little evidence of individuality. In some cases 

 we have evidence that structures in the center of a division figure 



