Chromatid. A threadlike strand formed from a chromosome during the early stages of cell division. Each chromosome divides along its length into two chromatids, which are at first held together at the centromere. They separate completely at a later stage. The DNA of the chromosomes reproduces itself exactly so that each chromatid has the complete amount of DNA and becomes a daughter of chromosome with exactly the same genes as the original chromosomes from which it was formed.

Chromatin.
A nucleoprotein, staining with basic dyes, and forming part of the chromosomes. It is made up of two proteins one of which is histone, DNA and RNA.

Choriod rete. A counter current arrangement of arterioles and venules which occurs in the eyes of teleost fish behind the retina.

Chromosome. A threadlike structure several to many of which are found in the nucleus of plant and animal cells. Chromosomes are composed of chromatin and carry the genes, which determine the individual characteristics of an organism. When the nucleus is not dividing, individual chromosomes cannot be identified with a light microscope. During the first stage of nuclear division, however, the chromosome contract and, when stained, can be clearly seen under a microscope.
Each consists of two chromatids held together at the centromere (see also meiosis; mitosis). The number of chromosomes in each cell is constant for and characteristic of the species concerned. In the normal body (see homologous chromosomes); in the gamete forming germ cells, however, the diploid number is halved and each cell contains only one member of each chromosome pair.

Chromonema.
An optically single thread forming an axial structure within each chromosome. The number of chromonemata (pl) in a chromosome has been found to be normally two.

Chromoplast. A plastic containing one or more coloured pigments.

Chromosomal mutation.
A mutation that occurs due to changes in he chrosomal structure or in the number of chromosomes.