28 THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



the chromosomes at this stage to assume characteristic configurations. 

 If the centromere is attached in the center, the chromosome will be V- 

 shaped; if attached between the center and the end, it will have a J- 

 shape; if attached at or very near the end, it will appear as a straight 

 rod. 



Telophase, the phase of reconstruction. When the chromosomes 

 reach the poles of the spindle they form a rather tight aggregation and 

 the cell is said to be at the beginning of the telophase. In many respects 

 this stage is an exact reverse of the prophase. The chromosomes be- 

 come longer and thinner as they become uncoiled and lose the matrix, 

 the spindle figure disappears, and the nuclear membrane reappears. 

 During these changes a cleavage furrow develops which gradually 

 pinches the cell into two parts and the process of mitosis is complete. 



Daughter Cells. We now have two cells which are in the interphase 

 again — cells which are identical and are usually known as daughter cells 

 at this time. The entire process of mitosis which we have described 

 consumes only about one hour as a rule. In so short a time there 

 can be little opportunity for cell growth, so that each of the two cells 

 which has been formed is only about one half the size of the original 

 cell. Yet — to emphasize it again— each of these cells has the same 

 number and kinds of genes as were present in the original, thanks to 

 the precision of the process of mitosis. This fact can be demonstrated 

 in a rather dramatic manner. If the original cell happened to be a 

 fertilized egg from which an animal, let us say a salamander, will de- 

 velop, then each of the daughter cells will normally go on to produce 

 one half of the body of the salamander. It is possible, however, to 

 separate the two cells experimentally, and then each cell will usually 

 form an entire salamander which will be no different from a salamander 

 which was formed from an unseparated fertilized egg. Furthermore, 

 these two salamanders will bear identical genes and will, therefore, show 

 identical hereditary characteristics. The same thing may be demon- 

 strated in human beings. Normally, a single fertilized egg will pro- 

 duce a single individual, but at times the early embryo may become 

 split into two parts and each part will go on to produce a complete 

 individual. This results in what we call identical twins because they 

 have identical genes and will always look very much alike. We should 

 point out, however, that there is another type of twins, the fraternal 

 twins, which originate as two fertilized eggs to begin with. They are 

 no more alike than brothers and sisters born at different times and may 

 be of different sexes. 



The process of mitosis is somewhat complex, but it is a very 

 efficient method of cell reproduction. It is the method by means of 



