366 THE WOKLD OF LIFE 



cell-substance. This serves to divide the chromatin elements into 

 two equal parts, to separate the resulting halves from one another, 

 and to arrange them in a regular manner. At the opposite poles 

 of the longitudinal axis of the nucleus two clear bodies — the 

 *' centrosomes/" each surrounded by a clear zone — the so-called 

 '' sphere of attraction '^ — now becomes visible (A to D, cs). They 

 possess a great power of attraction over the vital particles of the 

 cell, so that these become arranged around them in a series of 

 rays. At a certain stage in the preparation for division, the soft 

 protoplasmic substance of the cell-body as well as of the nucleus 

 gives rise to delicate fibres or threads ; these fibres are motile, and, 

 after the disappearance of the nuclear membrane, seize the chromo- 

 somes — whether these have the form of loops, rods, or globular 

 bodies — with wonderful certainty and regularity, and in such a 

 way that each element is held on either side by several threads from 

 either pole (B, C). The chromatin elements thus immediately be- 

 come arranged in a fixed and regular manner, so that they all come 

 to lie in the equatorial plane of the nucleus, which we may con- 

 sider as a spherical body." 



!N^ow follows another and even more remarkable stage in 

 the process, which is thus described : 



" The chromatin elements then split longitudinally, and thus 

 become doubled (B), as Fleming first pointed out. It must be 

 mentioned that this splitting is not caused by a pull from the pole 

 threads (spindle threads), which attach themselves to the chroma- 

 tin-rods on both sides ; the division arises rather from forces acting 

 in the rods themselves, as is proved by the fact that they are often 

 ready to divide, or indeed have already done so, some time before 

 their equatorial arrangement has taken place by means of these 

 threads. 



" The splitting is completed by the two halves being gradually 

 drawn further apart towards the opposite poles of the nuclear 

 spindle, until they finally approach the centre of attraction or 

 centrosome (D), which has now fulfilled its object for the present, 

 and retires into the obscuritv of the cell-substance, onlv to become 

 active again at the next cell-division. Each separated half of the 

 nucleus now constitutes a daughter-nucleus, in which it (the 



