PERMANENCE OF THE CHROMOSOMES. 59 



nuclear division to another. Rosenberg says nothing as to how the 

 chromosomes become connected with the spindle fibers. 



All the earlier evidence that the chromosomes are permanent struc- 

 tures in the cells of the higher plants and animals, except that of Rabl 

 and Flemming, has been developed from a study of the form, number, 

 appearance, etc., of the chromosomes themselves as they recur in each 

 dividing stage, and does not rest on any proof of the structural relations 

 in the nucleus by means of which, in all the manifold changes of fusion 

 and division, such permanence is assured. 



The nature of the mechanism by which the scattered chromatin 

 elements of the so-called resting condition are brought back into the 

 definite form and positions which the chromosomes occupy in the karyo- 

 kinetic figures has been left undetermined. Boveri (10, 11) puts the 

 question as to the possible nature of these structures clearly, but is com- 

 pelled to resort to numerous accessory hypotheses in order to account 

 for the regularity with which the chromosomes reappear in the same 

 positions and number after their apparent total disintegration in the 

 resting reticulum and the certainty with which each daughter chromo- 

 some is found attached to one, and only one, of the poles of the spindle. 

 He assumes a peculiar affinity on the part of each daughter element by 

 which it is predetermined that the fibers from a particular pole will 

 become attached to it, and, further, that there is an especial handle 

 (Henkel) on each daughter chromosome, by which alone the fibers may 

 become attached to it ; if a fiber from one pole has once gotten hold of 

 the handle on one daughter chromosome, those from the other pole 

 are excluded from it ; and, further, other fibers from the same pole are 

 unable to get hold of the handle on the other daughter chromosome of 

 the same pair, etc. It is hardly necessary to remark that the com- 

 plexity of the mechanism by which such a series of affinities and capaci- 

 ties could be brought into effective action is well-nigh inconceivable. 

 Boveri, in 1888 (10), and again in 1897 (11), decided against the pos- 

 sibility of a permanent connection between centers and chromosomes 

 in favor of the above hypotheses, and reaffirms his old position in his 

 most recent contribution on this subject. 



It is further interesting to note that the theory of the polarity of 

 the cell which has perhaps been most discussed in recent years — ^that 

 of Heidenhain (41) — leaves entirely untouched the question of the 

 organization of the nucleus, and further assumes not only that there is 

 no permanent connection of the central body with the nucleus, but that 

 the organization of the cytoplasm is entirely independent of that of the 



