xvn MYSTERY OF THE CELL 339 



The accompanying series of diagrams from Professor 

 Weismann's book already referred to are intended to show 

 the essential features of what takes place in a cell previous 

 to division, the detailed fibrous structure of the plasma 

 being omitted for the sake of clearness. It must be under- 

 stood, however, that much of what is described in the cells 

 is quite invisible even in the highest powers of the best 

 microscopes, owing to the fact that almost all the parts — the 

 fibrous network and granules in the plasma, as well as the 

 network in the nucleus — are transparent, and only become 

 visible by the use of various chemical reagents and dyes, 

 which stain some parts more than others and thus render 

 them visible. The parts of the nucleus which are thus 

 coloured and rendered visible are termed chromosomes or 

 chromatin. I will now quote Weismann's description of 

 what happens in such a cell. (Fig. i 10, p. 343.) 



" When the nucleus is going to divide, the chromatin granules, 

 which till then were scattered, become arranged in a line and form 

 a long thread which extends through the nucleus in an irregular 

 spiral (Fig. no A), and then divides into portions of fairly equal 

 length (the chro?noso?7ies). These have at first the form of long 

 bands or loops, but afterwards become shortened, thus giving rise 

 to short loops (B), or else to straight rods or rounded granules. With 

 certain exceptions the number of chromosomes which arise in this 

 way is constant for each species of plant or animal, and also for 

 successive series of cells. 



" By the time the process has reached this stage a special 

 mechanism appears, which has till now remained concealed in the 

 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 



