THE PRINCIPLES OF EMBRYOLOGY 469 



so give origin to reflex actions at different levels. Such neurones 

 would not be in the direct chain of either the afferent or efferent 

 neurones, and so not directly connected with the periphery, and 

 could therefore be removed without affecting the vitality of either 

 the efferent or afferent chain of neurones. In other words, the 

 vitality of the cells on the efferent side ought not to be dependent 

 on the integrity of the reflex arc. With regard to the development 

 of the anterior roots, Anderson has shown that this is the case, for 

 section of all the posterior roots conveying afferent impulses from 

 the lower limb in a new-born animal does not hinder the normal 

 development of the anterior roots supplying that limb. Also Mott, 

 who originally thought that section of all the posterior roots to a 

 limb caused atrophy of the corresponding anterior roots, has now 

 come to the same conclusion as other observers, and can find no 

 degeneration on the efferent side due to removal of afferent impulses. 



Again, the process of regeneration after section of a nerve is 

 not in favour of the neuroblast theory. There is no evidence that 

 the cut end of a nerve can grow down and attach itself to a 

 muscular or epithelial element without the assistance of a nerve 

 tube down which to grow. When the cut nerves connected with 

 the periphery degenerate, that applies only to the axis-cylinder 

 and the medullary sheath, not to the neurilemma ; the connective 

 tissue elements remain alive and form a tube into which the growing 

 axon finds its way, and so is conducted to the end-plate or end- 

 organ of the peripheral structure. 



Possibly, as suggested by Mott and Halliburton, the products 

 of degeneration of the axis-cylinder and medullary sheath stimulate 

 these connective tissue sheath-cells into active proliferation, and 

 so bring about the great multiplication of cells arranged as cell- 

 chains, which are so often erroneously spoken of as forming the 

 young nerves. These sheath-cells are then supposed to re-form 

 and secrete a pabulum which is important for the process of re- 

 generation of the down-growing axis-cylinder and medullary sheath. 

 Without such pabulum regeneration does not take place, as is 

 seen in the central nervous system, where the sheath of Schwann 

 is absent. 



A^ain, it is becoming more and more doubtful whether the 

 peripheral terminations of nerves are ever really free. As far as 

 efferent nerves are concerned the nervous element may entirely 



