2/8 GROWTH 



entiation in the transformation of such cells into permanent tissues, 

 constitute the essential features of growth. 



355. Cell Division. Division of a protoplast in the multiplica- 

 tion of cells is accomplished by a separation of the nucleus, cyto- 

 plasm and plastids into physiologically equivalent parts, which 

 are organized to carry out the functions of the original cell. Two 

 general methods, with regard to the action of the nucleus during 

 the process, may be distinguished, viz., mitosis and amitosis. 



Division of the cell with mitosis is characterized by a chemical 

 and physical transformation of the nucleus, in which the chro- 

 matin assumes the form of rods and increases in staining power. 

 The limiting membrane of the nucleus disappears, and much of 

 the cytoplasm is involved in the evolutions of the nucleus. The 

 chromosomes split longitudinally, and the halves separate and 

 collect in equal portions at poles of the spindle, being connected 

 by a number of interzonal fibers. This separation of the com- 

 ponents of the nucleus is generally followed by the division of the 

 cell by the formation of a plate or wall midway between the groups 

 of chromosomes and finally extending to the periphery of the 

 cell. Meanwhile the groups of chromosomes are organized as 

 daughter nuclei and quickly assume the structure of the original 

 nucleus (Fig. I3I). 1 



Division of the cell with direct separation of the nucleus into 

 two parts in amitosis has not been so thoroughly investigated. 

 It appears, however, that the nucleus is divided by a simple con- 

 striction cutting through the nucleus, which undergoes no struc- 

 tural changes of the chromatin, or reticulum, preliminary to this 

 process. Various intermediate stages between mitotic and ami- 

 totic division of the nucleus have been observed (Fig. 132). 



It is held by many writers that mitotic division is character- 

 istic of vigorous actively growing cells, and that the equal di- 

 vision of the chromosomes between the daughter nuclei is neces- 

 sary to insure the proper transmission of the parental qualities to 

 the two daughter cells, and that direct division is to be found 



1 See Wilson, E. B. The cell in development and inheritance. 65. 1900. 



