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 ; orig-in of the sympathetic nervous system ; biological 

 test of relationship. 



Criticism of Balanoglossus theory. — Theory of parallel development. — Impoi't- 

 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. 



