THE CELL AND PROTOPLASM. 119 



of the thread by splitting will give rise to a 

 double series of threads, each of which has 

 identical characters. Should the division occur 

 across the thread the two halves would be unlike, 

 but taking place as it does by a longitudinal 

 splitting, each unit in the thread simply divides 

 in half, and thus the resulting half threads each 

 contain the same number of similar units as 

 the other and the same as possessed by the 

 original undivided chromosome. This sort of 

 splitting thus doubles the number of chromo- 

 somes, but produces no differentiation of material. 



The next step in the cell division consists in 

 the separation of the two halves of the chromo- 

 somes. Each half of each chromosome separates 

 from its fellow, and moves to the opposite end of 

 the nucleus towards the two centrosomes (Fig. 

 31). Whether they are pulled apart or pushed 

 apart by the spindle fibres is not certain, although 

 it is apparently sure that these fibres from the 

 centrosomes are engaged in the matter. Certain 

 it is that some force exerted from the two centro- 

 somes acts upon the chromosomes, and forces the 

 two halves of each one to opposite ends of the 

 nucleus, where they now collect and form two 

 new nucleii, with evidently exactly the same 

 number of chromosomes as the original, and with 

 characters identical to each other and to the 

 original (Fig. 32). 



The rest of the cell division now follows 

 rapidly. A partition grows in through the cell 

 body dividing it into two parts (Fig. 32), the 

 division passing through the middle of the 

 spindle. In this division, in some cases at least, 



