Relation of Body Size to Muscle Cell Size and Number in the Chicken ,

THE size of an individual is a function of its cell size and cell number. Variation in body size depends upon the variation in one or both of these variables and a difference in extracellular material. Numerous genetic studies have investigated the nature of body size variation in birds and mammals. Estimates of genetic variance of total body size have been made from many of these studies but have not permitted the separation of the primary factors contributing to body size, ie. cell size and cell number. The understanding of growth and size variation would be markedly improved if its components could be analyzed independently. Robertson (1959), using the wing size of Drosophila melanogaster as an indicator of body size, demonstrated both phenotypic and genetic differences in cell number and size within the same wing size. Selection for small cell size was suggested as a direct means of breaking up…