Cell numbers during cleavage of the zebra fish egg.

The eggs of the zebra fish Brachydanio rerio (Hamilton-Buchanan) were used in these studies. Roosen-Runge (1936, 1938, 1939), Marrable (1959,1962a) and others have described various aspects of the biology of this fish when maintained in the laboratory. It is a freshwater tropical teleost, the females laying nonsticky, non-pigmented demersal eggs which undergo a regular and predictable pattern of cleavage; fertilization is external and the diameter of the spheroidal chorion enclosing each egg is about 800 /x. Breeding is normally promiscuous but when monogamy is imposed hundreds of fertile eggs can be obtained. Development is rapid: at 27-25±0-5°C. the first cleavage is 30 min. after fertilization; there are 32 cells after 90 min. and gastrulation takes place after about 4 hr.