THE MECHANISM OF MITOSIS ' HI 



fibrin-ferment. Attention may be called here to the fact, now definitely 

 determined by experiment,^ that cell-division may be incited by chemi- 

 cal stimulus. In most of the cases thus far experimentally examined 

 the divisions so caused are pathological in character, but in others 

 they are quite normal, as shown in Loeb's remarkable results on the 

 production of parthenogenesis in sea-urchin eggs by chemical stimulus, 

 as described at pages 2 1 5 and 308. While these experiments by no 

 means show that division is itself merely a chemical process, they 

 strongly suggest that it cannot be adequately analyzed without reckon- 

 ing with the chemical changes involved in it. 



Resume. A review of the foregoing facts and theories shows how 

 far we still are from any real understanding of the process involved 

 either in the origin or in the mode of action of the mitotic figure. The 

 evidence seems well-nigh demonstrative, in case of the mantle-fibres 

 and the astral rays, that Van Beneden's hypothesis contains an element 

 of truth, but we must now recognize that it was formulated in too 

 simple a form for the solution of so complex a problem. No satisfac- 

 tory hypothesis can, I believe, be reached that does not reckon with 

 the chemical changes occurring at the spindle-poles and in the nucleus ; 

 and these changes are probably concerned not only with the origin of 

 the amphiaster, but also with the movements of the chromosomes. In 

 cases where the centrosome persists from cell to cell we may perhaps 

 regard it as the vehicle of specific substances (ferments .'') which become 

 active at the onset of mitosis, and run through a definite cycle of 

 changes, to initiate a like cycle in the following generation ; and it is 

 quite conceivable that such substances may persist at the nuclear poles, 

 or may be re-formed there as an after-effect, even though the formed 

 centrosome disappears.^ In this consideration we may find a clue to 

 the strange fact — should it indeed prove to be a fact — that the cen- 

 trosome may divide, yet afterward disappear without discoverable 

 connection with the centrosomes of the succeeding mitosis, as several 

 recent observers have maintained. ^ When all is said, we must admit 

 that the mechanism of mitosis in every phase still awaits adequate 

 physiological analysis. The suggestive experiments of Biitschli and 

 Heidenhain lead us to hope that a partial solution of the problem may 

 be reached along the lines of physical and chemical experiment. At 

 present we can only admit that none of the conclusions thus far 

 reached, whether by observation or by experiment, are more than the 

 first naive attempts to analyze a group of most complex phenomena 

 of which we have little real understanding. 



1 See pp. 306, 308. 2 Cf. p. 215. ^Cf. p. 213. 



