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. 
Again, 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 
