Cell Division in Relation to Reproduction - 41 



a few special cases, are the only normal 

 methods of division that occur in nature. 

 Generally speaking, the body cells of the 

 organism are formed by mitosis, but the 

 eggs and sperm (in animals) and the spores 

 (in plants) are produced by meiosis. 



MITOTIC CELL DIVISION 



Mitosis, the more usual type of cell divi- 

 sion, represents one of the most intricate and 

 beautifully regulated processes in all the 

 realm of life. Essentially, it enables the cell 

 to produce two replicas of itself, the two 

 daughter cells, each possessing a full poten- 

 tial for perpetuating the unique character- 

 istics of its lineage. 



To some extent the successive stages and 

 events of mitosis can be observed sequentially 

 in the living cell. However, a full apprecia- 

 tion of the detailed pattern of changing 

 structure requires the use of cells that have 

 been fixed, stained, and subsequently studied 

 microscopically. In such cases, critical judg- 

 ment must be used in deciding upon the 

 proper sequence of the statically recorded 

 events, and always it must be realized that 

 only a derivative of the living structure is 

 under observation. 



Prophase. The mitosis of animal and plant 

 cells is fundamentally similar. There are 

 certain small differences, however, which will 

 be mentioned in the course of the following 

 general description. 



One very early sign that mitosis has started 

 is that the chromosomes, still within the con- 

 fines of the nuclear membrane, become easier 

 to identify individually. Each appears as a 

 long, slender thread (spireme stage) that 

 continues to become shorter and stouter and 

 to stain more and more intensely (Fig. 3-1, 

 prophases). In favorable cases, a spiral struc- 

 ture can be observed in each chromosome, as 

 is indicated diagrammatically in Figure 3-2. 

 The spirals appear to become more and more 

 tightly coiled as the chromosome progres- 

 sively shortens and thickens. Finally each 

 chromosome attains a very compact, in- 



tensely staining form (Figs. 2-11 A and 3-2). 

 Moreover, as soon as the chromosomes be- 

 come clearly visible, it can often be seen that 

 each consists of two parallel threads, lying 

 side by side. In fact, each prophase chromo- 

 some appears to be constituted, essentially, 

 of lu'o 1 parallel spiral strands of Feulgen 

 positive material (Fig. 3-2). 



Somewhere along its length, each chro- 

 mosome displays a more compact section, 

 which is designated as the kinetochore, or 

 centromere (Figs. 2-11 and 3-2). The kineto- 

 chore seems to represent a specific anchorage 

 point for the attachment of certain spindle 

 fibers. Usually the kinetochore occupies a 

 position more or less halfway between the 

 ends of the chromosome, although some- 

 times it is very eccentric, even to the point of 

 being terminally located. Soon after the 

 double structure of the chromosome becomes 

 discernible, it is possible to see that the kine- 

 tochore is also double. By the end of pro- 

 phase, in fact, the two halves of each chro- 

 mosome, which now are compact, rodlike, 

 and (usually) slightly bent at the locus of the 

 kinetochore, are separated slightly from one 

 another (Figs. 2-11 and 3-2). Now it can be 

 said that the process of chromosomal replica- 

 tion has been completed and that each origi- 

 nal chromosome has divided into two iden- 

 tical daughter chromosomes. 



While the chromosomes are shortening, 

 thickening, and becoming sharply delineated, 

 the nucleoli begin to lose their distinct out- 

 lines, and finally they fade from view com- 

 pletely. Also toward the end of prophase the 

 nuclear membrane begins to fade. Soon it 

 disappears, perhaps by fragmenting into 

 pieces that become dispersed (Fig. 3-2). 



Meanwhile dramatic events are going on 

 in the cytoplasm. Very early, while the chro- 

 mosomes are still elongate, the mitotic appa- 

 ratus (Fig. 3-3) appears. This consists of two 

 mitotic centers, between which there is a 



1 In some cases the coils themselves appear to be 

 coiled, so that a multiple, rather than a double, spiral 

 structure appears while the chromosomes are becom- 

 ing shorter and thicker during prophase. 



