Myomere numbers were counted in samples of reciprocal hybrids of lake and speckled trout. The fish which developed on speckled trout yolks, which are the smaller, had a lower mean myomere number. Myomere numbers of normal lake trout, speckled trout, and the F1 hybrid (lake trout egg) were compared with myomere numbers of their respective Siamese twins. An inverse relationship between myomere number and the degree of twinning was found in speckled trout but not in the lake trout or the hybrid. Since this relationship is not evident in fish developed on larger yolks it is assumed that there is a certain critical yolk size below which it becomes limiting and prevents the formation of the normal myomere complement.