CH. XIY.] REGENERATION OF NERVE 149 



the periphery increases with the increasing size of the developing 

 animal. 



I shall not fully discuss the pros and cons of this controversy, 

 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 years 

 been supported by a very remarkable ocular demonstration of its 

 truth. Mr Eoss Harrison of the Johns Hopkins University, Balti- 

 more, has 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 nerve-cells, and from these 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 a micro-millimetre in one 

 'or two minutes. Similar observations have since this been made in 

 the embryos of other animals. 



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 circum- 

 stantial 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, was formulated at a time when the 

 relationship of nerve-fibres to nerve-cells was not so fully recognised 

 as it is at present ; and the Wallerian doctrine may be accepted with 

 confidence to-day. It has, however, been questioned from time to 

 time, and the earliest to hold an opposite view was Vulpian. 

 Vulpian, working with Philippeaux, cut nerves in young animals, 

 excising long portions so as to prevent the two ends uniting. Some 

 months later they were surprised to find that a number of new 

 perfectly formed nerve-fibres had appeared in the peripheral segment, 

 and that this segment possessed the physiological properties of being 

 excitable and capable of conducting nerve impulses. To this 



