iv] NATURE OF THE SPINDLE 53 



definitely attached to the chromosomes, and as has already 

 been mentioned, the characteristic forms of the chromo- 

 somes in anaphase seem partly to depend on the point of 

 attachment of the fibre. (2) The spindle-fibres are some- 

 times quite definite in number, and so clearly marked that 

 they can be counted in transverse section; this is difficult 

 to understand if they are mere "chains of force." They 

 also may thicken as they shorten during the anaphase, and 

 the apparent complete disappearance of the mantle-fibres 

 in the telophase can be accounted for on the supposition 

 that they consist of a viscous fluid which contracts to a 

 small round drop. (3) The whole mitotic figure ("amphi- 

 aster") often shows signs of possessing considerable rigidity; 

 cases have been described in which when the cell was burst 

 the spindle was forced out as a whole into the surround- 

 ing fluid, where it could be seen rolling over and over and 

 persisting for some time before it broke up. Also several ob- 

 servers, for example, MORGAN (1910), and CONKLIN (1917), 

 find that in centrifuged eggs the spindle may be bent and 

 deformed, and can push aside yolk-granules and other dense 

 particles, and that in some cases it may be firmly attached 

 to the peripheral layer of the egg by the astral rays. These 

 observations seem definitely to contradict the chains of force 

 hypothesis, but it must be added that other investigators, 

 for example F. R. LILLIE (1909), maintain that the rays are 

 reformed after every change in the position of the poles, and 

 that centrifuging experiments support rather than disprove 

 the chains of force hypothesis. (4) In some cells, such as 

 leucocytes, the asters are said to persist and to form part 

 of the contractile mechanism of the cell. (5) It seems almost 

 certain that at times the rays from two asters may cross one 

 another, which should be impossible on the hypothesis of 

 chains of force. In view of these and similar facts it seems 

 probable that the supposition of a "dual force" as the cause 



