An analysis of muscle weight variations in the hindlimbs of Rana esculenta.

Dry weights of 24 muscles of the left and of the right hindlimbs in 63 frogs (Rana esculenta) are determined. By means of factor analysis is calculated which part of the variance in a muscle's weight can be accounted for by common factors and which part by the characteristics of the muscle itself. The main results are: 1. Body size (factor 1) accounts for more than 90% of the variance in all the heavier muscles and more than 64% in the eight lighter muscles. 2. Since only one single common factor is found, no separate groups of highly correlated muscle weights are present in the frog's hindlimb. 3. A decrease in the number of fibers is coupled with an increase in the residual variance of a muscle. 4. Left and right identic muscles follow the same variation-pattern. Two models for muscle development are discussed: 1. A model in which the innervation of the muscle determines the number of fibers. 2. A model in which clumping play an important role.