i6 Journal of Comparative Neurology. 



idea of the extreme simplicity where the spongioblasts and neu- 

 roblasts are clearly seen in their relation to the root and tracts. 

 In those cases where the sheaths of nerves are forming there is 

 no difficulty in distinguishing the elements of the latter from 

 the nuclei of the nerve fibres. 



The part played by these nuclei in regeneration is most 

 simply explained as a mere repetition of that part which they 

 originally took in the formation of the nerve. In the optic 

 nerve the formation of sheathes by interpolation of connective 

 matter is very readily seen in reptilia but the sheath material 

 cannot be compared with the neurons or their derivatives. In 

 mammalia and birds the relations are obscure. The chick is 

 one of the least satisfactory subjects for histogenesis in the 

 whole realm. 



DESCRIPTION OF PLATE II. 



Fij I. Root of oculo-motor nerve. Black snake embryo. 



Fig 2. Seventh nerve near its junction with the medulla of very young 

 Eutaenia embryo, a. a. case of subdivision of the neuroblasts. 



P^S- 3- Very early stage of Salamander larva, showing actual emergence 

 of neuroblasts from the medulla (on the right.) 



Fig 4. Isolated neurons of motor nerves in salamander larva. A double 

 Stain of hematoxylin and acid fuchsin. 



Figs. J, 6. Cross sections of ganglia with the emerging nerve, illustrating 

 the fact that the nuclei as well as the fibre, are independent of the wall and 

 that as a rule at this stage one does not encounter doi/i fibre and nucleus within 

 the sheath but one or the other as the section may pass. At a, however, the 

 fibre has been differentiated off from its nucleus whose protoplasm has disap- 

 peared. 



Fig. y. A neuron in which the thread is being differentiated off. 



Fig. 8. Fibres from the dorsal root crossed by others of the ventral root 

 passing to the dorsal muscles. 



Fig. g. Portion of a transection of the spinal cord of salamander larva 

 showing the distinction between spongioblast and neuroblast cells in the ventral 

 portion. 



Fig. 10. A few neurons from a nerve at its exit from the ganglion. At a 

 karyokinetic subdivision of a neuron ; at 3 a ganglion cell producing a fibre and 

 transforming into a nerve nucleus ; at <: an unaltered ganglion cell. The width 

 of the nerve is indicated by the outlines. 



Fig. II. One half of a section through the ventral part of the spinal cord 

 of salamander at the exit of a root. The median line is along a — a. The large 

 karyokinetic figures doubtless belong to the sheath ia which mitosis is very easily 

 observed. 



Fig. 12. Eighth nerve ? Several cells of the ganglion and the nidulus. 



