STRUCTURE AND ACTIVITIES OF CELLS 



The succeeding metaphase is the stage at which the longitudinally doubled 

 chromosomes are arranged on the equatorial plate (Fig. 2.10). The duality 

 that is such a conspicuous characteristic of a metaphase chromosome results 

 from the separation of the matrix after the genonemata have separated from 

 one another in the middle prophase. Each of the sister half-chromosomes in 

 the equatorial or metaphase plate contains one of the two genonemata found 

 in the prophase chromosome. The doubling of the chromosome in such a 

 way that sister half-chromosomes have equivalent genonemata is of great 

 theoretical significance (p. 190). The mechanism of distribution of chromo- 

 somes is regarded as a mechanism for the equal distribution of genes. 



The beginning of the anaphase is indicated by the separation of the halves 

 of each chromosome, a moving of one half-chromosome toward each centriole 

 or pole of the spindle (Fig. 2.10). What produces this movement is still un- 

 known, but it is initiated in the region of the kinetochore, now reduplicated. 

 As the half-chromosomes move toward the poles, careful examination reveals 

 that each contains two genonemata. This fact explains why two genonemata 

 occur in each prophase chromosome; the genonemata persist from one period 

 of division to the next. The exact time and the mechanism of replication of 

 the genonemata remains unknown. It can be stated, however, that one 

 genonema does not split to form two half-genonemata, each of which then 

 reforms the part it has lost. Instead, each genonema in some way serves 

 as a template upon which an exact duplicate is synthesized. When it is 

 completed, this new genonema becomes free from its pattern. 



As the chromosomes near the poles of the spindle, they come to lie very 

 close to one another. This marks the beginning of the telophase, during 

 which a new or daughter nucleus is formed from each clump of chromosomes 

 (Fig. 2.10). A nuclear membrane appears at the periphery of the chromo- 

 some group at each pole, and the members of the group begin to separate. 

 The chromatin progressively loses its capacity to stain, but not uniformly; the 

 thread-like genonemata stain after the matrix will no longer react with a dye. 

 Thus, the nucleus of a vegetative cell is formed, and one or more nucleoli 

 soon make their appearance. The centriole, meanwhile, has reduplicated, 

 and the spindle fibers begin to disappear. 



While the telophase of nuclear division is occurring, constriction of the 

 cytosome takes place in the plane of the equatorial plate of the mitotic 

 spindle. When constriction of the cytosome is complete, cell division is 

 finished; the entire process requires somewhat more than 30 minutes for its 

 completion. Two cells have been formed from one by a complicated process, 

 the most important aspect of which is the manner of distribution of the half- 

 chromosomes, each containing equivalent genonemata. Each new cell gets 

 exactly the same kind and amount of chromatic material. The essential 

 significance of mitosis is the equal qualitative and quantitative distribution 

 of the hereditary material. 



When the daughter cells enter the vegetative phase, they soon grow to the 

 size typical of their kind. It has been stated that this growth requires 



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