Herrick, TJie Development of Nejve- Fibres. 13 



of the eye-muscles, between the branches of the trigeminus and 

 the acoustic and faciahs, etc. Therefor Hensen maintains on 

 theoretical grounds that it is necessary to assume that 'the 7ierves 

 never grow out to tJieir terminations, but are ahvays in connection 

 with tJiem.' According to this view, which he endeavors to 

 support by observations, the embryonic cells are, for the most 

 part, united with one another by means of fine connecting 

 threads. He maintains that when a cell divides the connecting 

 thread also splits, and in this manner there arises an endless 

 network of fibres; Out of these the nerve-tracts are developed, 

 while other parts of the network degenerate." 



Respecting this theory it may be said that it has the obvi- 

 ous virtue of recognizing a difficulty which has been systematic- 

 ally underestimated or ignored. Furthermore recent discoveries 

 in lower vertebrates and worms show that the sensory nerves 

 primarily arise from the general epidermis which in the primi- 

 tive state consisted of a blending of neuro- and spongio-epithe- 

 lium. The ectoderm was homogeneous throughout. In the 

 worms the neuroblastic cells are not concentrated as perfectly as 

 in vertebrates and even in vertebrates the concentration varies 

 within wide limits. In Amphioxus the spinal ganglion lies 

 along the whole length of the dorsal root to its bifurcation. 

 We believe that the spinal ganglia of other vertebrates are to be 

 homologized with the disperse neuroblastic cells of the general 

 ectoderm and that the central trunk contains homologues of one 

 set of anastomosing branches, the peripheral trunk those of an- 

 other. 



It would seem impossible to doubt that the motor roots 

 actually arise in the cord and pass to their motor or trophic ter- 

 mini, though it has often seemed to the writer that the evidence 

 is that the formation of the specific muscle or gland is to be as- 

 cribed to the influence of the migrating nerve rather than that 

 the nerve "fundament" gropes blindly toward its destination 

 and plays a temporary game of hide and seek with the latter. 



Much more investigation in invertebrate neurology is needed 

 to enable us to grasp the morphological and physiological laws 

 which have been involved in the formation of the concentrated 



