348 MALE. [VoL. XIV. 
It is now generally believed, and to a great extent proved, 
that the ribs and muscles wander into the ventral walls of the 
embryo from the sclerosomes and myomeres on the dorsal side 
of the embryo. It is easy to demonstrate the growth of the 
ribs from the dorsal to the ventral side of the embryo, but it is 
not so easy to demonstrate the growth of many muscles from 
the muscle plates to their final position in mammalian embryos. 
With the growth of the ribs and the wandering of the muscles 
from the myotomes into the ventral wall of the embryo we 
have associated with them their nerves and blood vessels, which 
ultimately form the intercostal nerves and blood vessels respec- 
tively. The segmental arteries fuse at their tips early in their 
development immediately below the rectus abdominis to form 
the internal mammary and deep epigastric arteries. This chain 
of anastomosis is formed in a manner with that seen in the 
formation of the vertebral artery, as shown by His, Froriep, and 
Hochstetter. The primitive internal mammary and deep epi- 
gastric arteries, together with the rectus, at first lie immediately 
below the mammary line in pigs’ embryos, and all three, artery, 
muscle, and line, wander together towards the ventral middle 
line of the body. No doubt the same process takes place in 
the human embryo, but the mammary line lacks prominence 
here and cannot be used as a landmark. 
As the segmental nerves appear, each 1s tmmediately connected 
with its corresponding myotome, and all of the muscles arising 
from a myotome are always innervated by branches of the nerve 
which originally belonged to tt. 
This generalization is difficult to prove in all of its details, 
but all of the work in embryology, as well as a tabulation of our 
knowledge of motor nerve distribution, indicates that it must 
be true. The simpler muscles of the back, as well as the inter- 
costal, are innervated by their corresponding nerves. The 
muscles of the eye, of mastication, and of the face can be 
correspondingly grouped. The trapezius, latissimus dorsi, 
serrates anticis, and rectus abdominis wander great distances 
and carry with them their original nerves. Moreover, it would 
be practically impossible to explain why each muscle has its 
proper nerve supply, which is so evenly distributed in it, were 
