PROTOPLASM AND CELLULAR ORGANIZATION 



27 



Anaphase 



The daughter halves of the dupHcated 

 chromosomes now move to opposite ends 

 of the spindle (Fig. 9). Spindle fibers are 

 attached to the chromosomes at definite 

 points. The movement of the daughter 

 chromosomes is due to the contraction of 

 these spindle fibers. 



Telophase 



The daughter nuclei are now recon- 

 structed (Fig. 9). The chromosomes return 

 to the state in which they existed before 

 mitosis began, a nuclear membrane appears, 

 and the astral rays disappear. The cell body 

 divides into two by a constriction which 

 arises as a furrow at right angles to the spin- 

 dle. This furrow becomes deeper, until fi- 

 nally the cytoplasm is divided into two. 



The time required for nuclear and cyto- 

 plasmic division varies with the type of cell 

 and the temperature. At a temperature of 

 39° C, the mesenchyme cells of a chick 

 that were being grown in tissue culture di- 

 vided as follows: prophase, 5 to 50 minutes, 

 usually over 30 minutes; metaphase, 1 to 15 

 minutes, usually 2 to 10 minutes; anaphase, 

 1 to 5 minutes, usually 2 to 3 minutes; telo- 

 phase to cytoplasmic division, 2 to 13 min- 

 utes, usually 3 to 6 minutes; telophase re- 

 construction of daughter nuclei, 30 to 120 

 minutes; total 70 to 180 minutes. Cyto- 

 kinesis (cytoplasmic division) is usually 

 quite rapid. Moving pictures of dividing 

 cells prove that nuclear mitosis occupies 

 most of the time, whereas division of the 

 cytoplasm is accomplished very quickly. 



Many variations occur in the structure and 

 mitotic division of nuclei and in the division 

 of the cytoplasm. For example, in many of 

 the Protozoa and in certain cells of some 

 other animals, the mitotic apparatus is built 

 up within the nuclear membrane. Some pro- 

 tozoans and animals above the protozoans 

 in the scale of life produce a type of cell 

 that is capable of developing under certain 

 conditions into an organism like the parent; 



cells of this type are called gametes or germ 

 cells in contrast to the rest of the cells of 

 the body, which are known as somatic cells. 

 The description of mitosis presented here 

 applies to the division of body (somatic) 

 cells. Mitosis, during the development of 

 gametes, may differ in several very impor- 

 tant features from that of somatic cells. 

 These differences will be described later. 



Chromosomes 



Every species of animal has a definite 

 number of chromosomes that appear when 

 the cells of its body undergo mitosis. Thus 

 there are 4 in the nematode worm, Paras- 

 caris equorum; 8 in the fruit fly, Drosophila 

 melanogaster; and as many as 168 in the 

 brine shrimp, Artemia. An even number of 

 chromosomes is characteristic of most ani- 

 mals, but some forms have an odd number. 

 Chromosomes vary considerably in both size 

 and shape. Typically they are rodlike, but 

 some appear to be spherical. They may be 

 less than i-looo mm. or more than Y^q mm. 

 in length. The chromosomes that appear 

 during mitosis in the cells of an animal may 

 differ in size and shape; when such differ- 

 ences are visible they are not only charac- 

 teristic of all cells of that animal, but also 

 of the species. These differences are mostly 

 in length, the thickness usually being con- 

 stant. 



A chromosome is not a homogeneous 

 mass of dark-staining material as it appears 

 to be in many preparations, but it has a com- 

 plex structure. In the interphase (Fig. 10), 

 in some cases, it can be observed that the 

 chromosome consists of at least two thin 

 chromatin threads, the chromonemata (sin- 

 gular, chromonema * ) ; the chromonema is 

 the basic unit of the chromosome. The two 

 chromonemata are often so closely applied 

 to each other along their entire lengths that 



* The thread or strand visible in the light micro- 

 scope is called a chromonema, but the electron 

 microscope reveals that each chromonema is subdi- 

 vided into thin fibers. 



