the chromosomes 151 



The Individuality of the Chromosomes 



The view that the chromosomes are persistent as 

 individual structures in the cell has steadily gained 

 ground during the last twenty 3^ears. The process 

 of karyokinetic or mitotic division by means of which 

 at each cell division the halves derived from a length- 

 wise split of each chromosome are carried to opposite 

 poles, so that a genetic continuity is maintained be- 

 tween corresponding chromosomes (and parts of 

 chromosomes) in mother and daughter cells, has been 

 found to be almost universal in both plants and 

 animals. It is true that several instances have been 

 described in which the nucleus simply pinches into 

 two parts, and there can be little doubt that such cases 

 occur; but no one has been able to show in a convinc- 

 ing way that cells which have once divided in this 

 manner ever return to the regular process of karyo- 

 kinetic division. Case after case of amitosis that 

 has been described for the germ cells has been either 

 disproven, or found to rest on faulty observation, or 

 else to relate to cells like those of the egg coats that 

 take no part in the germinal stream. 



There are several observations that lead to the 

 view, at present generally accepted, that the chromo- 

 somes retain their individuality from one cell division 

 to the next. These may now be given. 



Diu'ing the resting stage the chromosomes spin out 

 in such a way that they appear to form a continuous 

 network in the nucleus. They can not be identified 

 individually during this period. Wlien the chromo- 



