IV] STRUCTURE OF THE CELL 169 



cell : of forces which, whatever may be their specific nature, at least 

 are capable of polarisation, and of producing consequent attraction 

 or repulsion between charged particles of matter. The opposing 

 forces which were distributed in equilibrium throughout the sub- 

 stance of the cell become focussed at two "centrosomes," which 

 may or may not be already distinguished as' visible portions of 

 matter ; in the egg, one of these is always near to, and the other 

 remote from, the "animal pole" of the egg, which pole is visibly 

 as well as chemically different from the other, and is the region in 

 which the more rapid and conspicuous developmental changes will 

 presently begin. Between the two centrosomes, a spindle-shaped 



Fig. 41. Caryokinetic figure in a dividing cell (or blastomere) of the Trout's 

 egg. (After Prenant, from a preparation by Prof. P. Bouin. ) 



figure appears, whose striking resemblance to the lines of force 

 made visible by iron-filings between the poles of a magnet, was at 

 once recognised by Hermann Fol, when in 1873 he witnessed for 

 the first time the phenomenon in question. On the farther side 

 of the centrosomes are seen star-like figures, or "asters," in which 

 we can without difficulty recognise the broken lines of force which 

 run externally to those stronger lines which lie nearer to the polar 

 axis and which constitute the "spindle." The lines of force are 

 rendered visible or "material," just as in the experiment of the 

 iron-fil ngs, by the fact that, in the heterogeneous substance of 

 the cell, certain portions of matter are more "permeable" to the 

 acting force than the rest, become themselves polarised after the 



