Life: Its Nature and Origin 31 



The living cell has a most precise life history, as is demonstrated 

 by the careful studies of Mazia (1956) on unicellular animals. 

 Typically the cell grows until it doubles its weight; then, without 

 further weight increase, it undergoes a period presumably occupied 

 by some sort of maturing activity; then it divides. If some cytoplasm 

 is removed from a newly full grown cell, this cell will grow some 

 more until its doubled weight is restored, then go into the dividing 

 sequence. However, Mazia found that there is a time point near the 

 beginning of the maturation period after which the cell, if injured, 

 will not stop to replace its weight, but will proceed with the division 

 process. In other words, there is a triggering point which starts the 

 actual division of the cell, and, after the trigger is pulled, the 

 process can go only toward completion. 



The physical process of division occurs in certain well-defined 

 stages (Fig. 12). The boundaries of the nucleus disappear and the 

 chromosomes, diffused within the nucleus during the growing 

 period, condense into compact threads (A and B). Simultaneously, 

 a mitotic center forms and divides, and a daughter center moves 

 to each pole (C). Each of these centers is a protein gel oriented 

 into a system of aster-like rays. During this development, the 

 chromosomes come to lie in a flat equatorial plane between the 

 poles, and the centromere of each chromosome is attached to a ray 

 of a mitotic center (D). These connecting rays become greatly 

 strengthened to form distinctive fibers. The entire system of fibers 

 is called the spindle. By this stage, each chromosome is double, 

 composed of two daughters. Next the daughter chromosomes 

 separate into two identical sets (E) and each set moves to a pole 

 (F). During this movement the chromosome-to-pole fibers shorten 

 and the equator-to-chromosome fibers lengthen; it is a moot point 

 whether the chromosome sets are pulled apart, pushed asunder, or 

 are moved by a combination of both. Finally the walls of the cell 

 pinch in (G); the mitotic centers and the spindle structures disap- 

 pear, the chromosomes loose their obvious identity to form a well- 

 defined nucleus, and the division is complete (H). Each daughter 

 cell now starts the cycle of growth all over again. 



The formation of the mitotic centers and the spindle is a real 

 puzzle. Mazia advanced the interesting hypothesis that these 

 centers and their rays and fibers arise through the formation of 

 bundles of connected protein molecules, each pair joined by a 

 sulphur-to-sulphur bond known as a disulphide bridge. He sug- 

 gested that, in a series of chemical reactions involving the sub- 



