Chordates: The phylum of animals whose primary axial skeleton consists temporarily or permanently of a notochord.
Chromatin: Same as basichromatin.
Chromosomes: The short threads or rodlike bodies into which the basichromatin of the cell-nucleus is aggregated during mitosis—each chromosome is segmented into granules called chromomeres—in its submicroscopic structure it consists of chain or linear series of genes (hereditary factors) representing characters linked together in heredity, each single chromosome being termed, on this account, a “linkage-group” by geneticists.
Ciliate: A protozoan whose motor-apparatus consists of cilia, i. e., hairlike protoplasmic projections capable of rapid and coördinated vibratile movement.
Cloaca: A common passageway through which the intestine, kidneys, and sex organs discharge their products,—it occurs in certain fishes, in amphibia, reptiles, and birds, and in a few mammals.
Coccyx: Lower extremity of the vertebral column in man.
Colloids: Insoluble gumlike substances, which will not diffuse through organic membranes.
Commensalism: The harmonious cohabitation of two organisms belonging to different species, where the relation is not necessarily beneficial nor necessarily harmful to either.
Crossover: The exchange or reciprocal transfer of whole blocks of genes from one homologous chromosome to the other, which sometimes occurs in synapsis, probably at the strepsinema-stage.
Crystalloids: Soluble substances, which usually form crystals and readily diffuse through organic membranes.