CHAPTER X 



THE BREAKDOWN OF THE GENETIC SYSTEM : 

 ADVENTITIOUS 



Abnormal Mitosis — Polymitosis — Failure of Pairing at Meiosis — Geno- 

 typically and Structurally Determined — Failure of Chiasmata and of Spindle- 

 formation — Non-Reduction and Diploid Gametes — Effects of Irradiation and 

 their Use in Experiment. 



Without contraries is no progression. 



Blake, Marriage of Heaven and Hell. 



I. THE BREAKDOWN OF MITOSIS 



(i) Introduction. So far we have considered the origin, evolution 

 and successful working of genetic systems of increasing complexity. 

 In them sexual reproduction has been built up on a foundation of 

 mitosis, meiosis and fertilisation. They show the operation of the 

 laws of heredity which have been made out by experimental breeding 

 in such systems. 



Structural and numerical hybrids are an exception. In them 

 the mechanism of reproduction and the laws of heredity partially 

 break down. It remains now to consider the conditions and 

 consequences of such a breakdown both in hybrids and elsewhere. 

 We find in the first place that not only meiosis, but also mitosis 

 may suffer abnormahty. This is particularly important because 

 here we know that the complex conditions of meiosis and particularly 

 of hybridity as affecting chromosome pairing are not concerned. 

 Genotypic and environmental factors are therefore readily distin- 

 guished from structural factors, and they wiU be specially considered 

 later. 



The occurrence of defective mitosis is, however, necessarily 

 limited, either to special tissues or to slight and harmless 

 irregularities, for this reason : any generally distributed and gross 

 abnormality of mitosis will immediately stop the development of 

 the organism, and ultimately kill it. 



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