170 ON THE INTERNAL FORM AND [ch. 



fashion of a magnetic or "paramagnetic" body, arrange themselves 

 in an orderly way between the two poles of the field of force, cling 

 to one another as it were in threads*, and are only prevented by 

 the friction of the surrounding medium from approaching and 

 congregating around the adjacent poles. 



As the field of force strengthens, the more will the hnes of force 

 be drawn in towards the interpolar axis, and the less evident will 

 be those remoter lines which constitute the terminal, or extrapolar, 

 asters : a clear space, free from materialised lines of force, may 

 thus tend to be set up on either side of the spindle, the 

 so-called "Biitschli space" of the histologistsf. On the other 

 hand, the lines of force constituting the spindle will be less con- 

 centrated if they find a path of less resistance at the periphery 

 of the cell : as happens, in our experiment of the iron-filings, when 

 we encircle the field of force with an iron ring. On this principle, 

 the differences observed between cells in which the spindle is well 

 developed and the asters small, and others in which the spindle 

 is weak and the asters enormously developed, can be easily 

 explained by variations in the potential of the field, the large, 

 conspicuous asters being probably correlated with a marked 

 permeability of the surface of the cell. 



The visible field of force, though often called the "nuclear 

 spindle," is formed outside of, but usually near to, the nucleus. 

 Let us look a httle more closely into the structure of this body, 

 and into the changes which it presently undergoes. 



Within its spherical outhne (Fig. 42), it contains an "alveolar" 



* Whence the name "mitosis" (Greek /j-ltos, a thread), applied first by Flemming 

 to the whole phenomenon. Kollmann (Biol. Centralbl. ii, p. 107, 1882) called it 

 divisio per fila, or divisio laqueis implicata. Many of the earher students, such as 

 Van Beneden (Rech. sur la maturation de I'oeuf, Arch, de Biol, iv, 1883), and 

 Hermann (Zur Lehre v. d. Entstehung d. karyokinetischen Spindel, Arch.f. mikrosk. 

 Anat. XXXVII, 1891) thought they recognised actual muscular threads, drawing 

 the nuclear material asunder towards the respective foci or poles; and some such 

 view was long maintained by other writers, Boveri, Heidenhain, Flemming, R. 

 Hertwig, and many more. In fact, the existence of contractile threads, or the 

 ascription to the spindle rather than to the poles or centrosomes of. the active 

 forces concerned in nuclear division, formed the main tenet of all those who dechned 

 to go beyond the "contractile properties of protoplasm" for an explanation of the 

 phenomenon. (Cf. also J. W. Jenkinson, Q. J. M. S. xlviii, p. 471, 1904.) 



f Cf. Biitschli, 0., Ueber die kiinsthche Nachahmung der karj'okinetischen 

 .Figur, Verh. Med. Nat. Ver. Heidelberg, v, pp. 28-41 (1892), 1897. 



