THE GENETIC SYSTEM 43 



the chromosomes constitutes the nucleus (what is called 

 the "resting nucleus"). 



5. In some organisms the entire set of 2« chromosome 

 vesicles can be dimly seen and counted, even in this resting 

 stage. In others this cannot be done. 



6. As any cell gets ready to divide again, a new, small 

 chromosome is laid down in each of the vesicles formed 

 by the previous chromosomes. 



7. When these new chromosomes first appear, they are 

 usually irregular granules, which gather into a thread, 

 having thickenings at intervals. In this condition they are 

 called the skein or spireme. 



8. These threads gather together and condense into the 

 chromosomes of the type seen at cell division (figures i, 



2,3). 



9. The remainder of each large chromosomal vesicle 



dissolves in the cytoplasm of the cell. Here it doubtless 

 produces chemical changes of importance. 



Thus after every cell division the chromosome takes up 

 a quantity of fluid from the cytoplasm, and later gives it 

 off again, doubtless in modified form. This is probably one 

 of the most important processes in development. It is pre- 

 sumably in this way that the chromosomes influence the 

 development and the characteristics of the organism. They 

 take up parts of the cytoplasm, change it, and give it off 

 again. In this way are probably made the different kinds 

 of tissues: muscles, nerves, bones, and the like. The chro- 

 mosomes are thus continually active in the process of 

 manufacturing the body (a matter to which we return 

 later) . 



10. While thus in vesicular form, and active in the physi- 

 ological processes of the cell, the chromosomes as such 

 have in most cases disappeared; they are no longer visible 

 (though in exceptional cases they can still be detected). 

 When later they reappear they show definite relations to 



