Charles R. Bardeen BoD 
nerve in the foot a mass of slightly differentiated tissue represents prob- 
ably the anlage of the musculature subsequently innervated by this 
nerve. 
The general relations of the plantar nerves at this period are strikingly 
similar in many ways to the plantar nerves in the crus of the lower mammals 
as recently pictured by McMurrich in this journal (04) and offer analogies 
with types there pictured by him for the lower vertebrates. The chief differ- 
ence between the nerves of the plantar side of the crus of mammals and that 
of the reptiles and amphibia is, as McMurrich has pointed out, the path for 
the fibers going to the medial side of the foot. In the mammals the nerve 
fibers take a course superficial to the deep muscles of the crus; in the inferior 
vertebrates they take a course in part beneath the deep muscles. In the 
amphibia and reptiles the nerve fibers for the medial side of the foot are 
more or less bound up with the nerves to the deepest muscles of the crus; 
in the mammals they are more or less bound up with nerves to the more 
superficial muscles. The nerve for the lateral side of the foot runs in most 
forms between the superficial and the deep musculature of the crus. 
In an embryo 20 mm. long (Plate IX, Figs. 5 and 6) the various 
muscles of the plantar side of the leg are much more highly differentiated 
than in the 14 mm. embryo. The soleus and gastrocnemius muscles have 
begun to extend tibialwards over the tibial nerve. The tendon of 
Achilles is well differentiated. The long flexor muscles are attached to 
an aponeurosis from which tendons extend to the digits. The poplteus 
muscle is clearly marked. The tibialis posterior is inserted into the side 
of the skeleton of the foot near the base of the first digit. In the foot 
the anlages of most of the intrinsic muscles can be distinguished but here 
the muscles are but incompletely differentiated. A group of muscles 
innervated by the lateral plantar nerve is to be distinguished from one 
innervated by the medial plantar nerve. 
The lateral and medial plantar nerves in this embryo are fused into 
a common trunk as far as the ankle. The nerves to the gastrocnemius- 
soleus group arise from the plantar surface of the tibial nerve in the 
thigh. To the deep surface of the soleus, however, a branch is given 
which arises from the deep surface of the tibial nerve in the leg. From 
this surface arise the nerves for the deep muscles of this region. In the 
foot the distribution of nerve branches to the muscles corresponds with 
that found in the adult. 
b. Embryonic Development and Variation in the Nerve Supply in the 
Adult of Each of the Chief Groups of Muscles. 
The development of the individual muscles of the back of the leg and 
foot can best be followed by taking them up according to the groups 
