Nucleus and Chromosomes 53 



chromosomes arc the first to be involved in the cellular death. Their 

 eventual disintegration is probably a consequence of cytoplasmic or 

 metabolic changes. A better understanding ot these ^vould be of great 

 physiological interest, for it appears that among the warm-blooded 

 species of vertebrates the chromosomes are unable to remain for more 

 than a few hours in a cell with arrested mitosis. Quantitative data 

 on this problem have been given in a preceding paragraph; it would 

 be necessary to know what the biochemical changes are which lead 

 to the destruction of the nuclear structures, and in what way this is 

 related to the prolongation of metaphase. 



Breakages such as transverse division of chromosomes in plants 

 have been reported. "^i A number of other observations have been 

 made along this line, but no tests have been performed to demon- 

 strate that colchicine increases their frequency. Broken chromo- 

 somes and fragments are observed in untreated cells. 



2.6-1: The destruction of chromosomes in Tubifex. Colchicine 

 is regarded as a destructive mitotic poison, leading to degenerative 

 changes of the nucleus in Tubifex,^'-^- 5^' -'^ as opposed to the inhibitive 

 mitotic poisons which prevent cell division mainly by disturbing the 

 spindle mechanism. Tubifex is very favorable for the study of early 

 development and cytoplasmic division, but the "numerous and very 

 small chromosomes are unfavorable for cytological analysis,'"''^ so this 

 mav ex])lain the great discrepancies between these findings and those 

 of -workers using different cells. 



\Vhen the egg of Tubifex is treated by colchicine during its first 

 cleavage, the spindle gradually fades away as it does in other objects. 

 Then the chromosomes become progressively pycnotic and lose all 

 visible structure. In the second cleavage, or after longer colchicine 

 treatments, a total disaj^jiearance of the chromosomes was observed. 

 5.3. 54. 5.-.. 9.T -phe cells became empty; no more nuclear material could 

 be stained by any method. More than seventy per cent of the eggs, 

 twelve hours after colchicine, had such empty cells. But a few hours 

 later, new nuclear structure appeared. First were seen protoplasmic 

 condensations which did not stain with the Feulgen reaction. Then 

 scattered Feulgen-i^ositive masses appeared in the cytoplasm (Fig. 

 2.11). They seemed structureless but bore some resemblance to the 

 small nuclei which are foiuid in the control eggs. It is suggested that 

 some synthesis of thymonucleic acid takes place in the cytoplasm. 



The accompanying Figure 2.11 shows pseudonuclei in Tubifex. 

 Among AmpJiibia after colchicine, podojjhylline, and ben/anthra- 

 cenequinone, evidence has been presented of a "nudtiplication of 

 nuclear material without mitosis."-^* 



One may, nevertheless, conclude that in animal cells other than 

 Tubifex, chromosomes disintegrate only when extensive degenerative 

 changes alter the whole cell. Contrary to plant cells, which may 



