710 DR ALEXANDER BRUCE AND DR JAMES W. DAWSON ON 
secreted by the myotome and the epithelium, which excite the amceboidism of the cone 
of growth. As early as 1892 Casat had compared the cénes de croissance with their 
terminal filaments to nerve pseudopodia which have amceboid movement and a certain 
impulsive force. 
CasaL explains HEtp’s pictures of the penetration of the terminal cone into the 
interior of cells of the cord and mesoderm (plasmodesmata) as due to the shrinkage of 
the tissue and the agglutination of the embryonic axons to the tissue elements. He 
thinks that HELpD’s view simply places the question of the orientation of the nerve paths 
and the peripheral connections on new ground. ‘ Au point de vue de cette théorie, la 
question se réduit & ces termes: en vertu de quelles conditions physico-chemiques se 
sont produits, dans certains endroits de l’embryon et avant l’apparition des axones, des 
chemins directs et parfaitement congruents entre tous les organes qui doivent ultérieure- 
ment contracter des connexions anatomiques et fonctionelles ?” 
Harrison (1905-1910) thinks that the attempt to answer the question of the 
development of nerves in normal embryos has been largely a matter of individual inter- 
pretation. He has therefore carried out a series of valuable investigations, along the 
line of experimental embryology, to eliminate all possible sources of error in coming to 
a conclusion as to the relation of the nerve fibres to the nerve cells. By his final work 
he claims to have conclusively established, on the basis of direct observation, the His 
teaching of the outgrowth of the nerve fibre from the central neuroblast. 
Harrison’s earlier embryological researches had led him to the conclusion that the 
sheath of Schwann cells arise from the neural crest, and, taking this as the starting-point, 
he tried first to answer the question of the source of the elements of the nerve fibres. 
In amphibian larvee, before any differentiation of nerve cells and fibres has occurred, he 
removed the source of the sheath of Schwann cells, z.e. the ganglion crest, and found 
that the motor nerves developed as naked fibres without sheath cells. Harrison then 
removed the source of the motor nuclei—z.e. the ventral half of the cord, leaving the 
dorsal portion of the cord and the ganglion crest—in order to answer the question: Can 
sheath cells without ganglion cells form the nerve fibres? The result was that sensory 
fibres and sheath cells appeared but no purely motor rami. Therefore, sheath cells by 
themselves cannot form fibres and ganglion cells by themselves can form naked axis- 
cylinders. 
Harrison next set himself to answer the question: What are the factors that 
influence the laying down of the nerve paths during embryonal development? Is the 
nerve fibre a product of the ganglion cell, or formed in stu in the peripheral path? He 
therefore first removed portions of the nerve centres and found that no peripheral nerve 
developed in relation to the absent ganglion cells. The second step consisted in the 
transplantation of undifferentiated portions of the nerve centres to abnormal positions 
of the embryo body, with the result that they gave rise to nerve fibres which 
followed paths in which normally no nerves were present. He concluded, therefore, 
that the nerve fibre is a product of the ganglion cell and not a mere activation of indif- 
