Comparative anatomical stu d ies of the innervation and musculature of the posterior limb were made on four northern fur seal fetuses, four adult cats and two kittens. The results may be summarized as follows:
1. The numbers of regional vertebrae in the fur seal and cat are different, but the total number of presacral vertebrae in both species is the same.
2. The femur and crus are relatively larger in the kitten than the fur seal, whereas the foot is considerably larger in the fur seal than the kitten.
3. On the basis of their innervation, the gluteus superficialis (maximus), caudofemoralis and caput longum m. bicipitis femoris appear to represent a closely related formation.
4. The sacroperoneus in the fur seal appears to be represented by the tenuissimus in the cat and by the short head of the biceps femoris in man.
5. Based on our observations of the nerve branching patterns and of dry muscle weights, it may be taken as a general rule that the distal displacement and size-increase of a muscle are accompanied by a high division in the plexus of the nerve supplying it.
6. The arrangement of the lumbosacral plexus t e nds to be conservative throughout different vertebrates, but the relative dry weights of muscles which are applied by this plexus vary from animal to animal according to their life environments.