38 THE THEORY OF THE GENE 



the chromosome theory. There is general agreement 

 amongst cytologists that when the chromosomes are set 

 free in the protoplasm they remain intact through the 

 entire period of cell division, but when they take up fluid 

 and combine to form the resting nucleus, it is no longer 

 possible to trace their history. By indirect means, how- 

 ever, it has been possible to get some evidence as to the 

 conditions of the chromosomes in the resting stages. 



After each division the individual chromosomes be- 

 come vacuolated as they come together to form a new 

 resting nucleus. They can be followed for some time, 

 forming separate compartments of the single nucleus that 

 re-forms. They then lose their staining quality and can 

 no longer be identified. When the chromosomes are again 

 about to appear, sac-like bodies are seen. This suggests, 

 if it does not prove, that the chromosomes have remained 

 in place during the resting stage. 



Boveri showed that when egg-cells of Ascaris divide, 

 the daughter chromosomes of each pair are pulled apart 

 in the same way, and often show characteristic shapes 

 (Fig. 25). At the next division of such cells, when the 

 chromosomes of daughter cells are about to reappear, 

 they show similar arrangements of their threads. The 

 inference is clear. The threads retain in the resting 

 nucleus the sliapes that they had when they entered the 

 nucleus. This evidence is favorable to the view that the 

 chromosomes have not passed into solution, and later 

 reformed, but have retained their intesfritv. 



Finally, there are cases where the chromosome num- 

 bers have been increased, either by becoming doubled, or 

 by crossing species with different numbers of chromo- 

 somes. There may be, then, three or four chromosomes of 

 each kind. The same number is retained as a rule through 

 all successive divisions. 



On the whole, then, while the cytological evidence does 



