Chain reaction. A chemical or atomic reaction in which the products of each state promote a subsequent reaction. Initially there is a slow induction period but as the reaction progresses the reaction rate is accelerated.

Chalaza.
1. A twisted stand of fibrous albumen in a bird’s egg that is attached to the membrane at either end of the yolk and thus holds the yolk in position in the albumen.

2. The part of a plant ovule where the nucellus and integuments merge.

Chalone. One of a group of chemicals occurring in living animal tissues which specifically inhibit Mitosis.

Chelonia. Tortoises and turtles. Order of reptiles with body enclosed in plates of bone usually covered by epidermal horny plates (tortoise-shell). Shoulder and pelvic girdles unique in being inside the ribs. No teeth.

Chemoautotrophic. (Chemosynthetic). Obtaining energy from a simple inorganic reaction, the nature of which varies according to species, e.g. oxidation of hydrogen sulphide to Sulphur by Thiobacillus. Several kinds or autotrophic bacteria are chemoautotrophic.

Chemoreceptor. Sensory receptor which detects and differentiates substances according to their chemical structure, by contact with their molecules, e.g. smell, taste.

Chemotaxis. Taxis, the stimuls having the form of a gradient of chemical concentration, e.g. movement of polymorph leucocytes towards bacteria in response to substance arising from reaction between bacteria and complement; or of fern spermatozoids to archegonia in response to malic acid diffusing from archegonial necks.