Charles R. Bardeen 3a 
In the mammals the homologies seem more certain. McMurrich considers 
the medial head of the gastrocnemius to be a muscle primitively distinct 
from the lateral head. He bases this conclusion on the fact that in many of 
the lower mammals each head forms a distinct muscle. The ontogeny of the 
muscle in man indicates that the two heads are derived from an anlage situ- 
ated on the fibular side of the leg. The twisting of the tendon of Achilles 
may be explained by the shifting which the muscle undergoes during onto- 
geny. Embryological development in man supports the idea advocated by 
McMurrich that the plantaris is a derivative of the deeper portion of the 
lateral head of the gastrocnemius. When absent it is likely that this separa- 
tion has failed to take place during ontogeny. In many mammals it is not 
differentiated (several edentates, carnivora, etc.) ; in others, especially in some 
rodents (Leche), it is highly developed. The soleus is considered by McMur- 
rich to be derived from the profundus musculature of the crus. It seems to 
me likely that the deep portion of the soleus, innervated by the distal nerve 
to that muscle may be thus derived from the profundus musculature although 
I have been able to distinguish no such special anlage in the development of 
the muscle in man. In the monotremes the soleus is bound up with the 
lateral head of the gastrocnemius. This arises from the epiphysial process 
of the fibula. It forms a part of the lateral head of the gastrocnemius in 
marsupials, in most edentates, in the chiroptera and galeopithecide, several 
carnivora, ungulates, and prosimians (Leche). The great number of mam- 
mals in which it is thus undifferentiated as a distinct muscle indicates 
strongly that its phylogenetic as well as its ontogenetic origin is, in the main 
at least, from an anlage common to it and the gastrocnemius. LHisler, g5, 
regards it as derived from the gastrocnemius lateralis. 
The variations in the muscles of the soleus-gastrocnemius group in man 
seem to be essentially due to a greater or less separation of the original an- 
lage into independent muscles. The fascial extension of the biceps, semi- 
tendinosus and gracilis, which I take to represent the plantaris superficialis 
medialis (McMurrich) of the amphibian crus, may be muscular instead of 
tendinous and may be somewhat fused to the gastrocnemius. 
2. Development of the Deep Muscles of the Back of the Crus. 
a. M. Popliteus—In an embryo 14 mm. long (Plate IX, Fig. 4) I 
have been unable to distinguish clearly a popliteus muscle. The anlage 
of the muscle doubtless lies in the dense tissue posterior to the tibial 
nerve and proximal to the anlage of the m. tibialis posterior. There is 
a differentiation of tissue there which indicates this. This tissue is out- 
lined in the drawing. In an embryo of 20 mm. (Plate IX, Fig. 6) the 
muscle is well defined, has the skeletal relations characteristic of the adult 
and at its distal border there enters a well marked nerve of supply. 
Schomburg does not mention this muscle in his article. 
In the adult the nerve usually enters the muscle near the center of its 
distal edge. Often some of the branches of this nerve extend into the 
