THE REPAIR OF A NERVE 417 



bridge between the central nervous system and the peripheral 

 organs, but exists from the very first, and in subsequent 

 development it merely undergoes elaboration, and increases in 

 bulk and in length as the distance from the central nervous 

 s^'stem and the periphery increases with the increasing size 

 of the developing animal. 



I shall not fully discuss the pros and cons of this con- 

 troversy, but only say that the available evidence appears 

 to me strongly in favour of the first of the two views, 

 and it has within the last few months been supported by a 

 very remarkable ocular demonstration of its truth. Mr. Ross 

 Harrison has, in the Johns Hopkins University, Baltimore, 

 actually seen the fibres growing outwards in embryonic 

 structures. Pieces of the primitive nervous tube which forms 

 the central nervous system were removed from frog embryos, 

 and kept alive in a drop of lymph for a considerable time ; 

 the cilia of the neighbouring epidermic cells remained active 

 for a week or more ; embryonic mesoblastic cells in the 

 vicinity were seen to become transformed into striated muscular 

 fibres, and there was therefore no doubt that even under 

 artificial conditions of this kind — rendered necessary for 

 microscopic purposes — life and growth were continuing. From 

 the primitive nervous tissue, and from this alone, nerve fibres 

 were observed growing and extending into the surrounding 

 parts. Each fibre shows faint fibrillation, but its most remarkable 

 feature is its enlarged end, which exhibits a continual change 

 of form. This amoeboid movement is very active, and it results 

 in drawing out and lengthening the fibre to which it is attached, 

 and the length of the fibre increases at the rate of about one 

 micro-millimetre in one or two minutes. 



I think these observations show beyond question that the 

 nerve fibre develops by the overflowing of protoplasm from 

 the central cells, and thus give us direct evidence in favour of 

 the view which most embryologists previously held mainly as 

 the result of circumstantial evidence. Such, then, being the 

 general state of our knowledge regarding the way in which 

 nerve fibres grow in the developing animal, it is not surprising 

 to find that the prevalent idea regarding their regeneration after 

 injury follows the same lines. The original teaching of the elder 

 Waller (1852) that regeneration occurs by fibres growing out 

 from the central stump into the peripheral segment of the nerve 



