STRUCTURAL UNITS IN CELLULAR PHYSIOLOGY 



203 



3. Tliat the chromosomes in tlie region 

 near the centromere are capable oL" liberat- 

 ing material Avhich disaggregates the long 

 particles in its immediate vicinity. 



4. That there are progressive chemical 

 changes of a cj^clical nature which first 

 tend to lengthen and later to break up 

 these long particles. 



The mechanism of cell division would 

 then proceed as follows (Fig. 3). From 



Fig. 3. Meclianism of cell division. 



the centrosome a wave of orientation would 

 result first in a physical aster (a). The di- 

 vision of the centrosome, due to instability 

 in the field of these forces, would next lead 

 to the formation of a spindle (b). At the 

 same time, the centromeres having divided, 

 small negative spindles would be formed 

 around each of them. These negative spin- 

 dles would be then drawn into the equa- 

 torial plane of orientation parallel to the 

 main spindle (c). In cases where the main 

 .spindle is not formed, subsidiary spindles 

 would simply be arranged in parallel. As 

 the elongation of the particle proceeds, 

 both the positive and the negative spindles 

 would grow in length, driving the now di- 

 vided chromosomes farther and farther 

 apart, and leaving an area of parallel ori- 

 ented material between them (d). Finally, 

 the long particles would break up into 

 short ones and the process would pass into 

 the next resting phase (e). The formation 



of a dividing cell wall would seem to be 

 closely linked with the existence of the 

 long particle field, as it tends to grow per- 

 pendicular to the fiber direction. The evi- 

 dence for tlie truth of this picture is still 

 very scanty. The existence of a positive 

 tactoid is, however, fairly safe, as it is 

 marked in living cells by its birefringence. 

 For the negative tactoid there is much less 

 evidence ; some photographs do, however, 

 seem to show that the remaining fibers at 

 anaphase seem to be located not on the 

 lines joining the chromosomes but in the 

 areas between them, as the existence of 

 negative factoids would demand. The 

 whole picture at this stage is much too 

 crude and takes no account of the very 

 large variety of methods of division found 

 in cells, but it may be a useful starting 

 point for renewed study of the details of 

 these processes. 



It is possible to extend the application 

 of the idea of long-range forces to cover 

 even more complicated intercellular proc- 

 esses, namely those connected with the 

 pairing and movement of the chromosomes 

 themselves. But here, naturally, explana- 

 tions are necessarily even more speculative 

 than in the case of the spindle mechanism. 

 Chromosomes are known to vary in length 

 by a factor of the order of 20 at various 

 stages of mitosis. In its most extended 

 form in the resting stage or in the salivary 

 gland, the chromosome appears to be in 

 every respect a typical fibrous protein, mi- 

 cromolecular or perhaps microtactoid, 

 probably composed of alternate protein 

 and nucleoprotein components. The char- 

 acter of any structure having a linear ar- 

 rangement in a solution will be such that 

 any change, particularly any change on 

 the interface, must cause the structure to 

 lengthen or shorten. If it has, besides, an 

 internal structure of an unsymmetrical 

 kind, this shortening will take the form of 

 spiralization. Wlierever conditions favor 

 small surfaces, the spirals will be close, 

 and vice versa. 



The interaction of different chromosomes 

 can be at any rate partially accounted for 

 by the long-range forces due to their ionic 

 atmospheres. As has already been said, 



