100 THE STORY OF THE LIVING MACHINE. 



to generation, and indeed that the chromosomes 

 of the egg contain all the traits which the parent 

 hands down to the child. Now, if this chromatin 

 thread consists of a series of units, each repre- 

 senting certain hereditary characters, then it is 

 plain that the division 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 sim- 

 ply 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. 



FIG. 31. FIG. 32. 



FIG. 31. Stage showing the two halves of the chromosomes sepa- 

 rated from each other. 



FlG. 32. Final stage with two nucleii in which the chromosomes 

 have again assumed the form of a network. The centrosomes 

 have divided preparatory to the next division, and the cell is 

 beginning to divide. 



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 



