554 



CONTINUITY OF LIFE 



approximate stages. The so-called "resting" 

 phase, or interphase, indicates the stage 

 when there is little or no apparent activity 

 in the nucleus. Actually, however, it is dur- 

 ing this phase that growth is going on at 

 a rapid rate and metabolism may in reality 

 be at its peak. Under the microscope the 

 chromosomes in this phase are sacculated 

 and the chromatin appears to be dispersed. 

 The first signs that cell division is immi- 

 nent are the separation of the centrioles 

 (not seen in whitefish cells) and the gath- 

 ering of the chromatin into what appears 

 to be a fine thread. This is called the early 

 prophase. Later, when the centrioles have 

 reached the opposite poles of the cell, the 

 nuclear membrane fragments and disap- 

 pears. Simultaneous with this event, astral 

 rays radiate out from the centrioles and 

 fibers extend from one centriole to the 

 other, forming a spindle. By this time, the 

 chromatin has thickened and the chromo- 

 somes first appear visibly paired, although 

 duplication has occurred at some earlier 

 stage. The chromosomes move about rather 

 haphazardly at this time but finally settle 

 at or near the middle of the cell. This ter- 

 minates the prophase and introduces the 

 metaphase in which the chromosomes align 

 themselves along the equatorial plane of 

 the spindle, each pair being associated with 

 a single spindle fiber. The paired chromo- 

 somes then are seemingly pulled toward 

 opposite poles by the spindle fibers. There 

 is considerable controversy as to just how 

 they reach tlie poles, but all observations 

 show that they are attached to the spindle 

 fibers and probably manage to reach the 

 poles of the cells through the influence of 

 these structures in some manner not thor- 

 oughly understood. This migration stage 

 is designated the anaphase. Finally a new 

 nuclear membrane forms around the chro- 

 mosomes which assume their characteristic 

 dispersed condition observed at the begin- 

 ning in the interphase. This is the telophase. 

 The cytoplasm cleaves and finally the 

 daughter cells are completely formed, ready 



to undergo a growth period before the next 

 division is initiated. 



While there are individual variations 

 among animal cells, the process of mitosis 

 is essentially the same in all cells. Many 

 plant cells do not have centrioles and they 

 separate the cytoplasm in a manner differ- 

 ent from animal cells, but division of the 

 nuclear elements is the same. 



Factors regulating mitosis 



Cells divide at varying rates in the differ- 

 ent tissues of the body, some undergoing 

 mitosis at extremely short intervals whereas 

 others divide only rarely. For example, 

 sperm cells are produced continuously at 

 a tremendous rate in the mammalian testis, 

 millions every day. Likewise, red blood 

 cells are manufactured at the rate of 2/2 

 million per second, meaning that an equal 

 number of mitoses must occur. On the other 

 hand, some cells such as those in the bones 

 and nervous systems are rarely replaced, 

 hence mitoses occur only rarely in these 

 tissues. All of the cells in an embryo are 

 undergoing rapid cell division but as the 

 organism matures the tempo gradually 

 slows down. Biologists have been and still 

 are puzzled over the problem as to what 

 controls this rate of cell division. 



It is common knowledge that cells in the 

 region of an injury divide more rapidly 

 than those in the uninjured area. Obviously, 

 the increased rate is essential if the wound 

 is to be closed. Some biologists have been 

 able to demonstrate the presence of a 

 "wound hormone" produced by injured and 

 dead cells which is tliought to stimulate 

 mitoses. This has been demonstrated in 

 plants and it is thought that a similar mech- 

 anism may operate in animals. 



There is also a purely physical reason 

 why cells must divide when they reach a 

 certain size. If small pieces of cytoplasm of 

 an amoeba are cut away periodically the 

 cell fails to divide at all. It seems that a 

 certain ratio between the surface area of 

 the nuclear membrane and the amount of 



