350 MALL. [Vou. XIV. 
main guide is its nerve. This has been plainly indicated recently 
by Nussbaum,! and in my studies in embryology and anatomy 
I cannot find a single instance to contradict this idea. 
The same generalization, no doubt, applies also to the sensory 
nerves, which are distributed in a segmental way to the integu- 
ment. Their first distribution would then be to the regions 
immediately over the spinal ganglia, and as the skin shifts in 
its development it carries with it its original nervous supply. 
That the nerves cannot be distributed to any great extent 
without some guide is further demonstrated by all experiments 
on regeneration, which show that they have great powers of 
growth if they are guided in their distribution by a canal or by 
the connective tissue sheath of some degenerated nerve. 
Although it is impossible to prove at present that all of the 
skeletal muscles arise from muscle plates or their corresponding 
coelomic diverticula into the branchial arches, the generalization 
that they do arise from the mesoderm segments is based upon 
sufficient observation to make it highly probable. In sharks 
the coelom extends into the branchial arches and into the 
myotomes ; in man the cavity in the muscle plates is never con- 
nected with the coelomic cavity, while in the branchial arches 
no such cavity exists. Yet we do not hesitate to assert that 
the condition found in sharks is the primitive, while that in 
man is due to secondary changes. Of course the problem 
is more difficult when the fate of the plates is to be studied. In 
the sharks, where they are definite, it is easy to show that all 
of the skeletal muscles of the body arise from them, while in 
man it is extremely difficult to show from what myotome any 
given muscle arises. The first difficulty encountered is that in 
the head (with the exception of in the occipital region) there is 
no grouping of the mesoderm in the branchial arches, while in 
the extremities the myotomes lose their sharp outline and 
appear to blend with the surrounding mesenchym. This fact 
has been sufficiently emphasized by Paterson? a number of 
years ago, and recently has been used by Fischel? and by Har- 
1 Nussbaum, Verhandl. d. Anat. Ges., 1894-18096. 
2 Paterson, Quar. Journ. of Micro. Sci., N.S., vol. xxviii, p. 109. 
8 Fischel, Alorph. Jahro., Bd. xxiii, p. 544. 
