2 CELL GENETICS 



be separated from it without altering its essential character. 

 Protoplasm is an organ of behaviour, not a chemical entity. 



In certain organisms no differences of structure have yet been 

 detected between different parts of the protoplasm. In most 

 bacteria this is perhaps due to their small size. In the 

 Cyanophyceae and larger bacteria, differences have been found, but 

 their meaning is still doubtful. Elsewhere it is always possible to 

 distinguish between a small dense nucleus and the rest of the living 

 substance, which is called cytoplasm. In the cytoplasm other 

 bodies can be very generally made out ; they are known generically 

 as chondriosomes and plastids. They are very widely recognised, 

 but their relation with one another in different groups of organisms 

 are not always clear. It is known, however, in the flowering plants 

 that plastids differ in their potentialities for developing pigment 

 and sometimes transmit these differences permanently and indepen- 

 dently of other genetic influences. This can be shown by breeding 

 experiments, but not by direct observation (Renner, 1934). The 

 nucleus, on the other hand, can be seen in all stages of development. 

 It is always carried by the germ cells of both parents in sexual 

 reproduction and is the only structure of which this is known. It 

 is recognised customarily by its characteristic method of propaga- 

 tion. All nuclei arise by division of a pre-existing nucleus into two. 

 To do this, the mother-nucleus resolves itself into a number of 

 double bodies, the chromosomes, whose halves separate to form two 

 daughter nuclei which are exactly equivalent, and all the descendants 

 of a nucleus derived in this way have the same complement of 

 chromosomes. This process is known as mitosis. 



All organisms may be said to arise from bodies of protoplasm with 

 single nuclei. Each of these is described as a single cell. In the 

 lowest organisms with nuclei, the Protista, growth of this cell is 

 followed by its division, together with that of its nucleus, by mitosis, 

 and the separation of the daughter cells. The organism thus remains 

 tmicellular and every mitosis is an act of reproduction, but instead 

 of one individual begetting another, one individual so far as we can 

 see simply becomes two. The external simplicity of its organisation 

 is not always associated with simplicity of structure within the cell. 

 Indeed, while the Protista embrace the simplest organisms they also 



