CHAPTER XV 
FINAL REMARKS 
Problems requiring investigation— 
Giant nerve-cells and giant-fibres; their comparison in fishes and in arthro- 
pods ; blood- and lymph-corpuscles ; nature of the skin; origin of system of 
unstriped muscles; origin of the- sympathetic nervous system ; biological 
test of relationship. 
Criticism of Balanoglossus theory—Theory of parallel development.—Import- 
ance of the theory advocated in this book for all problems of Evolution. 
THE discussion in the last chapter on the “ Principles of Embryology ” 
completes the evidence which I am able to offer up to the present 
time in favour of my theory of the “Origin of Vertebrates.” There 
are various questions which I have left untouched, but still are well 
worth discussion, and may be mentioned here. The first of these is 
the significance of the giant nerve-cells and giant nerve-fibres so 
characteristic of the brain-region of the lower vertebrates. In most 
fishes two very large cells are most conspicuous objects in any 
transverse section of the medulla oblongata at the level of entrance 
of the auditory nerves. Each of these cells gives off a number of 
processes, some of which pass in the direction of the auditory nerves 
and one very large axis-cylinder process which forms a giant-fibre, 
known by the name of a Mauthnerian fibre. Each Mauthnerian 
fibre crosses the middle line soon after its origin from the giant-cell, 
and passes down the spinal cord on the opposite side right to the 
tail. Here, near the end of the spinal cord, it breaks up into smaller 
fibres, which are believed by Fritsch and others to pass out directly 
into the ventral roots to supply the muscles of the tail. Thus Bela 
Haller says: “The Mauthnerian fibres are known to give origin to 
certain fibres which supply the ventral roots of the last three spinal 
nerves, so that their terminal branches serve, in all probability, for 
the innervation of the muscles of the tail-fin.” They do not occur in 
the eel, according to Haller, or in Silurus, according to Kolliker, 
