PROTOPLASM AND CELLULAR ORGANIZATION 



25 



This is called mitosis. The two daughter 

 cells proceed to grow, and they in turn di- 

 vide, and so on, generation after generation. 

 Many cells, however, notably those formed 

 during the development of eggs, grow very 

 little or not at all during the period between 

 successive divisions. Why cells divide when 

 they do is not known, but the relative quan- 

 tities of nucleoplasm and cytoplasm are usu- 

 ally maintained in each kind of cell. It has 

 been suggested that when the cytoplasm 

 reaches a volume too great for the nucleus, 

 division begins. 



Interphase {"resting") cell 



A cell that is not undergoing division has 

 been called a "resting" cell. However, it is 

 anything but a resting cell in the true sense 

 of the word. It is carrying on all the life 

 processes of any living cell, and a more ap- 

 propriate name for it is an interphase cell. 

 This period in the life of a cell is one in 

 which no visible structural changes are tak- 

 ing place in the nucleus. This stage is not 

 considered one of the phases of mitosis, al- 

 though there is no sharp line of demarca- 

 tion between the late telophase and inter- 

 phase as shown in Fig. 9. The description of 

 the generalized animal cell (Fig. 6) is that 

 of a typical interphase cell. 



Alitosis 



Cell division involves a series of processes 

 of considerable complexity and of great 

 significance. The nucleus divides first and 

 then the cytoplasm. Constant reference to 

 Fig. 9 will make clear the following brief ac- 

 count of mitosis in a typical cell. Four stages 

 are recognized. 



1. Prophase: the mitotic figure arises and each 

 chromosome appears to be split longitudi- 

 nally (Fig. 9); actually, each chromosome 

 has duplicated itself. 



2. Metaphase: the duplicated chromosomes be- 

 come located in the equatorial plane of the 

 mitotic figure (Fig. 9). 



3. Anaphase: the halves of the duplicated 

 chromosomes separate and move as two 



groups to opposite ends of the mitotic 

 spindle (Fig. 9). 

 4. Telophase: two daughter nuclei are formed 

 and the cell body divides (Fig. 9). 



These 4 stages will now be described in more 

 detail. 



Prophase 



The chromatin in the interphase nucleus 

 (Fig. 9) may appear to be in the form of 

 isolated granules or a network of granules. 

 However, there is good evidence to indicate 

 that the chromatin is actually in the form of 

 fine threads which are much coiled. The 

 modern view is that the so-called granules 

 are actually a mass of very fine coils. What 

 may appear to be chromatin granules of the 

 interphase nucleus of some cells now can be 

 seen as distinct threadlike structures (Fig. 

 9). These threads (chromonemata) are 

 really double (Fig. 10). The chromonemata 

 go through a process of spiralization, which 

 is accompanied by a shortening and thicken- 

 ing of the chromosome. These chromosomes 

 are characteristic in size, shape, and number, 

 depending on the species of animal to which 

 the dividing cell belongs. While this is hap- 

 pening, a halo of radiating fibers appears 

 around the centrosphere, thus forming an 

 aster. The two centrioles then separate and 

 migrate to opposite ends of the cell, each 

 with an aster about it (Fig. 9). Between the 

 asters and the nuclear membrane, a number 

 of fibers become visible in fixed material 

 (Fig. 9). The nuclear membrane breaks 

 down and disappears; and the fibers, extend- 

 ing from the asters across the nuclear space, 

 form a spindle. 



Metaphase 



During this phase of mitosis the duplicated 

 ("split") chromosomes become located in 

 the equatorial plane of the spindle (Fig. 

 9). The two daughter chromosomes pro- 

 duced from one are identical with each 

 other and with the chromosome from which 

 they developed. 



