THE MECHANISM OF MITOSIS jy 



be observed, and the chromosomes proceed to the extreme limit of 

 the spindle-poles and appear actually to pass into the interior of the 

 huge reticulated centrosphere. I cannot see how this behaviour of 

 the chromosomes is to be explained as the result solely of a con- 

 traction of fibres stretching between them and the centrosphere. 

 It is certain, moreover, that another factor is at work. Throughout 

 the anaphases, the centrosphere steadily grows until, at the close, 

 it attains an enormous size (Fig. 37), and its substance differs chem- 

 ically from that of the rays, for after double staining with Congo 

 red (an acid aniline) and haematoxylin it becomes bright red while 

 the ra^'s are blue. It seems probable, therefore, that the movements 

 of the chromosomes are affected by definite chemical changes occur- 

 ring in the centrosphere, as Butschli^ and Strasburger^ have main- 

 tained ; and it is possible that the substance of the spindle-fibres 

 may be actually taken up into the centrosphere, and the chromo- 

 somes thus drawn towards it. Strasburger has made the interesting 

 suggestion, which seems well worthy of consideration, that the move- 

 ments of the chromosomes may be of a chemotactic character. In 

 any case, I believe that no satisfactory hypothesis can be framed 

 that does not reckon with the chemical and physical changes going 

 on in the centrosphere, and take into account also the probability 

 of a dynamic action radiating from it into the surrounding struct- 

 ures. Van Beneden's hypothesis is probably, in principle, correct ; 

 but, as Boveri himself admits in his latest paper ('95), it seems cer- 

 tain that other factors are involved besides the contractility of the 

 achromatic fibres, and the mechanism of mitosis still awaits adequate 

 physiological analysis. 



2. Division of the CJiromosomes 



In developing his theory of fibrillar contractility Van Beneden 

 expressed the view — only, however, as a possibility — that the 

 splitting of the chromosomes might be passively caused by the con- 

 tractions of the two sets of opposing spindle-fibres to which each is 

 attached.'^ Later observations have demonstrated that this sugges- 

 tion cannot be sustained ; for in many cases the chromatin-thread 

 splits before division of the centrosome and the formation of the 

 achromatic figure, — sometimes during the spireme-stage, or even in 

 the reticulum, while the nuclear membrane is still intact. Boveri 

 showed this to be the case in Ascaris, and a similar fact has been 

 observed by many observers since, both in plants and in animals. 



The splitting of the chromosomes is therefore, in Boveri's words, 



1 '92, pp. 158, 159. - '93, 2. 3 'S;, p. 279. 



