Abstract
Thexhromosomal complement of the Echidna is 64 in the female and 63 in the male which is the heterogametic sex. This indicates an XX female, XO male sex-chromosome mechanism. The position of the monotreme karyotype relative to those in birds, reptiles and mammals is discussed. Mitotic yields for varied conditions of culture of peripheral leucocytes and for the hypotonic treatment of testis material are given.