14 CELL GENETICS 



be due to heredity or due to the environment, or a particular problem 

 is said to be " genetical " or " physiological," the statement is 

 merely relative and depends on convenience of comparison with 

 some ideal or " normal " heredity or environment. This implication 

 is often forgotten because the " normal " condition seems so obvious. 

 Yet it is not at all obvious in cases where a condition of revealing 

 the character is, as is often the case in considering chromosome 

 behaviour, differentiation within the organism. An abnormal 

 property of part of the cells in one organism may be said to be 

 determined by an environmental difference between these cells and 

 the rest of the organism ; it may also with equal correctness be said 

 to be determined by a genetic difference between this organism and 

 other organisms. For example, the formation of testes and ovaries 

 by different cells of the same hermaphrodite is primarily a genetic 

 property, just as is the formation of testes and ovaries in different 

 males and females of a species, but while the first is immediately 

 determined by a developmental difference (differentiation) the 

 second is immediately determined by a genetic difference. 



Take another instance of great importance in cytology where, 

 after a number of mitotic divisions, a diploid nucleus suddenly 

 divides by meiosis. The difference in behaviour of the nucleus at 

 different times shows it to be subject to differentiation, as are the 

 larger structures. It is also subject to genetic control, as shown by 

 comparison with other organisms having no meiosis. Where, on 

 the other hand, meiosis occurs immediately after fertilisation, it is 

 equally possible to regard it as determined directly by the diploid 

 condition and not by genetically controlled differentiation since all 

 the following mitoses might equally take the form of meiosis in a 

 diploid nucleus. 



Now, our task is to find the materials whose properties answer 

 to the abstract description of the genotype. They must be embodied 

 in " permanent " structures ; that is to say, the structures themselves 

 must be stable, and they must be capable of reproducing themselves 

 identically. 



These permanent structures are found in the nucleus at the 

 resting stage and in the chromosomes during mitosis. The nucleus 

 and its chromosomes alone amongst visible structures are handed 



