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 dongitudzna/ 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. 
146 
