CHAPTER IV 



CELL MULTIPLICATION AND THE CELLULAR 

 STRUCTURE OF ORGANISMS 



BEFORE undertaking the study of the multicellular organisms 

 we must study in detail the process by which cells multiply. 

 We have already seen that the Amoeba, Paramecium, and other 

 single-celled animals and plants have the power of dividing. 

 Indeed all active, growing cells have the power of multi- 

 plying by division. Although division seems a very simple 

 process, in reality it is unexpectedly complex. The internal 

 changes in the cell during division have been made out only 

 by long study. While they differ in many small details, 

 all cells agree in certain broad general facts. The process 

 known as karyokinesis or mitosis (Gr. mitos = thread) is alike 

 in outline in most cells and is as follows : 



CELL DIVISION OR KARYOKINESIS * 



The Resting Cell. In Figure 36^4. will be seen a cell in 

 the condition of rest, before it has passed into the stage of 

 division. It will be noticed that the centrosome is in the form 

 of two minute granules, and that the chromatin inside of the 

 nucleus is in the form of a diffused network. No other factors 

 need concern us at the present time. 



1. Prophase. The first stage in the division involves both 

 the nucleus and the centrosome. In the nucleus the chromatin 

 assumes the form of a long thread sometimes known as the 

 spireme. This condition, however, is only preliminary to the 

 breaking up of the thread into a number of short pieces which 

 are called chromosomes (Gr. chroma = color + soma body) ; 

 Fig. 36 B. The number of chromosomes which arise in the 

 nucleus varies with different organisms but is constant for each 

 species of organism and is always an even number. In the 



*As here described karyokinesis applies chiefly to animal cells. 

 85 



