THE CEIJ, AND ITS IJFE. 27 



nucleus is seen to divide, and the two parts move toward 

 the opposite ends or poles of the cell. Just what these 

 centrosomes are is still an unsettled question. Radiating 

 from these centrosomes there soon appear little delicate 

 threads, believed by some observers to have been derived 

 from the achromatin of the nucleus; by others, from the 

 protoplasm of the cell body. The threads between the 

 centrosomes seem to be continuous, that is, extending 

 from one centrosome to the other, and thus forming a so- 

 called "spindle." The continuity of these threads is 

 questioned by others, who believe that they extend no 

 further than the " equatorial plate." In the meantime the 

 nucleus undergoes changes by which the chromatin of the 

 nucleus appears as a more or less coiled thread, which soon 

 breaks up into a number of V-shaped loops called chromo- 

 somes. The number of these chromosomes varies for the 

 different species of animals or plants, but in the same 

 species it is practically constant. These chromosomes 

 arrange themselves in the region of the "equatorial plate." 

 A longitudinal splitting of each chromosome doubles the 

 original number of Vs. Half of the V's now move toward 

 the one pole, the other half to the opposite pole. 

 Whether the spindle threads serve in any way to draw or 

 guide the individual V's is an interesting but unsolved 

 question. When the chromosomes have arrived at their 

 destination the individual V's again fuse together, and soon 

 two nuclei have replaced the original one. The cell proto- 

 plasm now seems to arrange itself with reference to the two 

 new nuclei, and a constriction of the old cell wall, or the 

 formation of a new wall along the line of the equatorial 

 plate finally produces two separate and distinct cells, which 

 in turn, by repeating this same process, may produce two 

 more, etc. The extreme care in dividing the nucleus 

 (which is the bearer of heredity) shows that nature takes 

 every precaution to give to each daughter-cell an exact 

 proportion of all those attributes which heredity passes 

 from one generation of cells or individuals to the next. 



