IV] AND STRUCTURE OF THE CELL 299 



capable of polarisation, and of producing consequent attraction or 

 repulsion between charged particles. The opposing forces which 

 are distributed in equilibrium throughout the cell become focused 

 in two "centrosomes*," which may or may not be already visible. 

 It generally happens that, in the egg, one of these centrosomes is 

 near to and the other far from the "animal pole," which is both 

 visibly and chemically different from the other, and is where the 

 more conspicuous developmental changes will presently begin. 



Between the two centrosomes, in stained preparations, a spindle- 

 shaped figure appears (Fig. 88), whose striking resemblance to the 



Fig. 8». Caryokinetic ligure in a dividing cell (or blastomere) of a trout s egg. 

 After Prenant, from a preparation by Prof. Bouin. 



lines of force made visible by iron-filings between the poles of a 

 magnet was at once recognised by Hermann Fol, in 1873, when he 

 witnessed the phenomenon for the first timef. On the farther 

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

 which we se^m to recognise the broken lines of force which run 

 externally to those stronger lines which lie nearer to the axis and 

 constitute the "spindle." The lines of force are rendered visible, 

 or materialised, just as in the experiment of the iron-fihngs, by the 

 fact that, in the heterogeneous substance of the cell, certain portions 



* These centrosomes are the two halves of a single granule, and are said (by 

 Boveri) to come from the middle piece of the original spermatozoon. 



t He did so in the egg of a medusa {Geryon), Jen. Zeitschr. vii, p. 476, 1873. 

 Similar ideas have been expressed by Strasbiirger, Henneguy, Van Beneden, 

 Errera, Ziegler, Gallardo and others. 



