Spindle and Cytoplasm 87 



graied, and are closch coordinated processes/^ One cannoi always 

 mark the separation between the jjrocesses. For this reason and per- 

 haps others, biologists use the term mitosis as completely synonymous 

 with cell division, when mitosis is only one aspect of a dividing cell.*" 



A\'hen colchicine acts during a dixision. the significance ol what 

 has been noted lor mitosis and cell di\ision becomes apparent. The 

 multijjlication of chromosomes continues in the presence of the drug 

 at a certain concentration, xvhereas the total absence of spindle fibers 

 prevents the movement of chromosomes to the respective poles. In- 

 hibition of fibers has one drastic effect on the cytoplasmic phases of 

 cell division: the cytokinetic processes are completely eliminated. 

 Among animal cells the cleavage jirocesses are somewhat specific and 

 respond to colchicine in a unique fashion. These aspects are discussed 

 in the next section. In plants no cell plate is formed, and phragmo- 

 plasts are prevented. For organization purposes these are discussed 

 separately from animal cells. 



5.7— /; Cleavage processes in annuals. Marine eggs have been sub- 

 jects for studying the mechanism of cell di\ ision since the pioneering 

 work of Hertwig, Boveri, and \\'ilson. The sea urchin, Arbacia 

 pnn( tiilata, was therefore a logical selection for Nebel and Ruttle"- 

 when, in 1937, they wanted to analyze more completely the activity of 

 colchicine. They established that 10 ^Ai" concentrations block cleavage. 

 Even a concentration of ().00()2 M inhibits cytoplasmic division'' if 

 applied 22 minutes after fertilization at 22° to 24.4°C. At this time 

 eggs are in prophase, metaphase, or early anaphase, and spindle mecha- 

 nisms are inhibited or destroyed by colchicine (Fig. 3.3) . 



If nuclear mitosis passes a certain stage, clea\age is not stoj^jjed 

 by these concentrations. Therefore, a critical point is reached beyond 

 which destruction of spindle apjiarently has no effect. These points 

 emphasize a close integration between nuclear mitosis and cytokinesis. 



20. 97, 98 



Specific objectives were outlined to determine precisely up to 

 Avhat stage or stages in the mitotic cvcle treatment was effective in 

 blocking cleavage and at Avhich stage colchicine Avas no longer effec- 

 tive. The results showed that suppression of cleavage by colchicine 

 follows a particular course on the basis of fertilized eggs of Arbacia 

 pun( tiiJata? The eggs were allowed to stand 10 minutes after fertili- 

 zation: then different lots were placed in colchicine at 2-mimae inter- 

 vals dining a 60-minute period. B\ this test, a lapse of 22 minutes be- 

 tween fertilization (22° to 24.4°C.) and the addition of colchicine was 

 found as the critical period, because cleavages were not blocked after 

 that time (Fig. 3.3) . 1 he mitotic stages most generally present at 

 this time were prophase, metaphase, and possibly early anaphase, each 

 of which was affected b\ colchicine. 14iese stages regularlv precede the 



