300 ON THE INTERNAL FORM [ch. 



of matter are more "permeable" to the acting force than others, 

 become themselves polarised after the fashion of a magnetic or 

 "paramagnetic" body, arrange themselves in an orderly way 

 between the two poles of the field of force, seem to cling to one 

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

 friction of the surrounding medium from approaching and con- 

 gregating around the adjacent poles. 



As the field of force strengthens, the more will the lines 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 fines of force, may 

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

 ''Biitschfi space" of the histologistsl. On the other hand, the lines 

 of force constituting the spindle will be less concentrated if they 

 find a path of less resistance at the periphery of the ceU : 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 greatly 

 developed, might easily be explained by variations in the potential 

 of the field, the large, conspicuous asters being correlated in turn 

 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. 



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

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

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

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

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

 mikrosk. Anat. x,xxvii, 1891) thought they recognised actual muscular threads, 

 drawing the nuclear material asunder towards the respective foci or poles; and 

 some such view of Zugkrdfte was long maintained by other writers, by Heidenhain 

 especially, by Boveri, Flemming, R. Hertwig, Rhumbler, 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 declined to go beyond the "contractile properties 

 of protoplasm" for an explanation of the phenomenon (cf. J. W. Jenkinson, 

 Q.J. M.S. XLViii, p. 471, 1904. See also J. Spek's historical account of the theories 

 of cell-division. Arch. f. Entw. Mech. xliv, pp. 5-29, 1918). 



t Cf. 0, Biitschli, Ueber die kiinstliche Nachahmung der karyokinetischen 

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



