BASES or CHANCE 



the shrimp Artemia salina) or where sexual differentiation is replaced 

 by hermaphroditism (as in nematodes and molluscs). 



Structural Change 



Apart trom clianges in the number of whole chromosomes, 

 changes also occur within the chromosomes, structural changes. Wc 

 have already an indication of such changes in the normal crossing- 

 over at meiosis; but of course crossing-over is reciprocal and can do 

 no more than rccombine differences that are already there. The 

 results of new changes that have taken place in the resting nucleus 

 we might see at the ensuing mitosis, especially at metaphase. But 

 examination of hundreds of individuals and thousands of mitoses 

 will fail to discover any spontaneous changes within chromosomes. 

 Only in abnormal individuals or under abnormal conditions, as 

 from sudden changes of temperature, do we find the chromosomes 

 breaking up, and then the break-up is usually frequent and extensive. 

 Its common characteristic is the breakage of the chromosomes or 

 chromatids during the resting stage, with results that are seen at the 

 following metaphase. Such spontaneous breakage has been described, 

 for example, in the pollen grains of Tulipa fragrans. Breakage can 

 also be induced by X-rays or other ionizing radiations, or by chemical 

 poisons such as mustard gas. The same description, however, can 

 apply to all three, since they follow the same rules. 



The simplest change, and the one which precedes all others, is 

 breakage of the chromosome fibre into two or more parts called 

 fragments. The breakage occurs in the resting nucleus. Now the 

 chromosome reproduces, its single thread becoming doubled, in the 

 resting nucleus. The breakage therefore affects either the whole 

 chromosome before its reproduction (B"), or only one of its two 

 daughter chromatids after its reproduction (B'). B" always gives 

 two kinds of fragments, one with the centromere and one or more 

 without (Fig. 20). 



Acentric fragments cannot move spontaneously on the spindle: 

 they lag and are usually left out of the daughter nuclei, like unpaired 

 chromosomes at meiosis. They degenerate in the cytoplasm. Evi- 

 dently they cannot develop a new centromere. It seems, indeed, that 

 we have to look upon the centromere as a specific and permanent 



100 



