88 Colchicine 



usual furrowing process by about lU to 14 minutes. Therefore, after 

 the critical mitotic stage, anaphase was passed, the furrowing pro- 

 cess started, and after that point colchicine did not inhibit cleavage 

 of the cell into two parts. 



Similar results were obtained from tests-" using the starfish. 

 Asterias forhcsii; the sea urchin, Arbacia punctulata; sea urchins from 

 Bermuda. Tripneustes esculentus and Lytechinus variegatits: and the 

 sea slug, Chroinodoris sp. In all cases, the key for inhibiting cleavage 

 was anaphase. The concentrations varied, but otherwise the general 

 plan was very similar for all tests. Once the eggs passed metaphase, 

 cleavage could not be altered by dosages of colchicine that destroyed 

 the mitotic spindle. If threshold concentrations were used at meta- 

 phase, furrowing almost divided the egg, and a regression then set in. 

 This showed that the final closing of cytoplasm is distinctly a process 

 dependent ujjon the spindle. Cases such as these emphasize the inter- 

 dependence between karyokinesis and cytokinesis as processes of cell 

 di\ision that invohe nucleus and cytoplasm. 



Cytological evidence for action by colchicine is obtained from the 

 lakelike bodies appearing where astral rays and spindle fibers nor- 

 mally should be found'' (Fig. S.?>) . One lake body indicates pro- 

 phase; two, one on either side of a clumped mass of chromosomes, 

 point to action at metaj^hasc: and two clusters of chromosomes can 

 be taken as evidence for disturbed anaphase. All these prevented 

 cleavage. 



Furrowing is dependent upon viscosity changes, and once processes 

 begin, apjjarently colchicine does not stop cleavage. In an effort to 

 correlate such changes with the cleavage process, centrifugal exjjeri- 

 ments were run, but not all results are in agreement." Ihe addi- 

 tional evidence ''" for viscosity or rigidity relationships and nuclear 

 mitosis as well as cytoplasmic division are discussed under the mecha- 

 nisms in the last chajjter. 



A demonstrated fact emerges that cleavage is averted if achro- 

 matic figures are destroyed before a certain mitotic stage has been 

 reached. Of course, concentration \arial)ilities are important, but 

 the blocking process appears to be an "all-or-nothing" effect; there- 

 fore, either nuclei divide and there follows a cytoplasmic di\isi()n, f;r 

 an arrested mitosis precludes daughter cell formation. For example, 

 chromosomes, scattered as a result of colchicine, form micronuclei. 

 and no cytoplasmic di\ ision takes place."*- i*'- ^^ On the other hand, re- 

 covery among a numljer of star mctaphases may eventually lead to 

 the cytoplasmic division, because spindle inactivation is not complete. 



Depending ujK)n (oiuentration, cleavages may be retarded or 

 stoj^ped (Fig. 3.3) . The germ cell of Triturus helveticiis L. does 

 not cleave if a 1:500 colchicine solution is used.''-' Regeneration of 



