Chap. II. THE MECHANISM OF HEREDITY
A cell is a lump o f jelly-like substance known as protoplasm. In the middle of the cell there is a darker mass of substance called the nucleus, the remainder of the cell going by the name of cytoplasm. And when cells divide to form new cells, as they continually doa process known as mitosis-the nucleus, if it is examined under the microscope, is seen to form itself first o f all into a number of long rod-like bodies known as chromosomes. The exact number of chromosomes differs in different species, but it is the same in every member of the same species.