Section HI 



CHAPTER 3 



Cell Division 



HANS RIS 



INTRODUCTION 



Growth usually is connected with the for- 

 mation of new cells. The ways in which new 

 cells originate have been intensively inves- 

 tigated and much debated ever since the 

 cellular nature of organisms was realized. 

 Though Trembley had already illustrated 

 binary fission of a diatom in 1748, the sig- 

 nificance of this process was first realized by 

 von Mohl in 1837 (Baker, '51). Yet not 

 until the middle of the century did it be- 

 come generally accepted that cells originate 

 only through division of pre-existing cells, 

 mainly because of the influence of Schleiden 

 and Schwann who vigorously fought for 

 their theory of free cell formation. With the 

 introduction of fixatives and stains, the par- 

 affin sectioning technique and improved mi- 

 croscopes, more detailed study became pos- 

 sible. The first suggestion of the complexity 

 of nuclear division appeared in a paper by 

 Schneider in 1873 who noticed the forma- 

 tion of chromosomes and their separation 

 into two groups which formed the daughter 

 nuclei. In a few exciting years following this 

 the essential features of cell division in plants 

 and animals were discovered through the 

 work of Flemming and Strasburger, van 

 Beneden, Fol and Biitschli, O. and R. Hert- 

 wig and Boveri. 



In 1879 Flemming described the longitud- 

 inal division of the chromosomes and the 

 separation of the halves into the two daughter 

 cells. The appearance of these basophilic 

 threads in the nucleus and their exact longi- 

 tudinal division was the most striking aspect 

 of cell division and Flemming therefore 

 called it "mitosis" (mitos = thread). The 

 direct simple fission of nucleus and cyto- 

 plasm was thought to be another mode of cell 

 proliferation and named "amitosis." With 

 the establishment of the chromosome theory 

 of inheritance at the turn of the century 

 the complexity of nuclear organization was 

 realized and the significance of the beautiful 



precision of mitosis became clear. More 

 gradually we have become aware of the com- 

 plexity of cytoplasmic organization because 

 it is more subtle and more varied. Today 

 the cell appears as a complex system, a kind 

 of hierarchy of more or less autonomous 

 components. In the chromosomes we have 

 the highest concentration of the factors that 

 are responsible for specificity in the cell, and 

 their irreplaceability is expressed in the ex- 

 treme care taken in their exact duplication 

 and distribution during the reproduction of 

 the cell. It is not surprising that the mecha- 

 nisms by which the cell accomplishes this 

 have been in the center of interest so far. 

 There exist in the cytoplasm, however, sys- 

 tems of at least partial autonomy and the 

 ways of their reproduction and distribution 

 must be more intensively investigated. For 

 the student of development and differentia- 

 tion particularly they may prove of even 

 greater interest than the behavior of the 

 chromosomes. 



The division of the nucleus, however, is 

 the central process around which cell re- 

 production is organized and it is better known 

 than the reproduction of the cytoplasmic 

 systems. The following discussion will, there- 

 fore, be organized mainly around nuclear 

 behavior. No attempt will be made to review 

 the large literature in the field. The dis- 

 cussion may be regarded as rather a per- 

 sonal and therefore somewhat biased essay 

 on some of the essential problems of cellular 

 reproduction. 



DESCRIPTION OF MITOSIS IN THE 

 WHITEFISH BLASTULA 



From algae to orchids and amoeba to man 

 the essential processes of cell division are 

 remarkably similar. We can describe, there- 

 fore, the division of one cell and thereby il- 

 lustrate the fundamental strategy in all. 

 Only bacteria and blue-green algae still 



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