572 Stewart Paton 



by the method the details of which are given in Section 3, the neu- 

 lotibrillation in the ventral roots always proeeeds from the apex 

 towards the base of cell and never in the opposite direction. 

 The apparent outgrowth of fibrils in connection with the cells of 

 Beaud may be explained by the dififerentiation taking- place in 

 situ in the axons as these lengthen. Various theories may be 

 brought forward to explain the manner in which the connections are 

 established between the cells. Held is of Ihe opinion that the 

 fibrils grow out of one cell and at some intermediate point, not yet 

 determined, fuse with those projected from a second cell. In the 

 majority of instances it seems to me probable that a fusion first 

 takes place between the undifferentiated plasmodesmata connecting 

 the cells and then neurofibrillation follows in these bridges. A 

 strong point in favor of this hypothesis seems to be found in 

 the case of the cells in the ventral horns where, as already 

 pointed out the first fibrils appear at the distal end of the axon. 

 If it is supposed that an actual growth takes place the diffi- 

 culty of explaining how the fibrils at the same time grow out 

 towards a second cell as well as inwards in the direction of 

 the basal process is again presented. 



Even in the case where connections are established between 

 cells lying very far apart, it seems to be not at all ^improbable that 

 undifferentiated bands of protoplasm at first form a span in which 

 the neurofibrils are secondarily differentiated. How^ far these bridges 

 are the projected arms of neuroblasts, whether intermediate links 

 are made up of protoplasm ejected from cells of a different type, 

 as well as the determination of the point at which fusion takes place 

 are all ([uestions requiring fnrther study. All reference to the bundles 

 of fibrils which constitufe true nerves has been purposely postponed, 

 as it is impossible to understand the way in which they are formed 

 without an accurate knowledge of the successive stages in their 

 development. 



In Fig. 20, Piate 24 a well developed brauch of the N. ventralis 

 spinalis is shown with many elongated nuclei in close contact with 

 this structure. In Selachians many of the cells, which eventually 

 take part in the formation of the sheath, pass out of the cord by 

 way of the ventral roots, so that Harrisun's well known experiment 

 if repeated would, at least in this species, give different results. 

 Within this particular nerve the longitudinal direction in which the 



