Nucleus.

 The nucleus consist of a more compact mass of protoplasm, separated from yhe cytoplasm by nuclear membrane which is also selectively porous, allowing substances to escape from the nucleus into the cytoplasm or substances to pass into it. The nucleus controls the cell and all its activities. Without a nucleus the cell would die.

The nucleus contains many protein-rich threads lying in a nuclear sap. In the ‘resting cell’ the threads are collectively spoken of as chromatin. These threads or chromosomes are vital to the everyday activities of the cell and are responsible for determining the hereditary characteristics of the human body. On the chromosomes in linear arrangement sit the genetic or hereditary determinants, the genes. The number of chromosome in a body cell is constant for a particular species of organism. In man there are twenty three pairs of forty six chromosomes.