The Making of Species 



These chromosomes appear, under ordinary 

 circumstances, to be joined together end to end, 

 and then look like a rope in a tangle. 



When a cell is about to divide into two, these 

 chromosomes become disjoined and can then be 

 counted, and it is found that each cell of each 

 species of animal or plant has a fixed number of 

 these chromosomes. Thus the mouse and the 

 lily have twenty- four chromosomes in each cell, 

 while the ox is said to have sixteen of them per 

 cell. 



When a cell divides into two, each of these 

 chromosomes splits by a longitudinal fissure into 

 two halves, which appear to be exactly alike. 

 One-half of every chromosome passes into each 

 of the daughter cells, so that each of these is 

 furnished with exactly half of each one of the 

 rod-like chromosomes. In the cell division, 

 which takes place immediately before the male 

 gamete or generative cell meets the female 

 gamete, the chromosomes do not divide into 

 equal halves, as is usually the case. In this 

 division half of them pass into one daughter cell 

 and half into the other daughter cell, so that, 

 prior to fertilisation both the male and the female 

 gametes contain only half the normal number of 

 chromosomes. In the sexual act the male and 

 the female chromosomes join forces and then the 

 normal number is again made up, each parent 

 contributing exactly one half. 



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