104 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



making their appearance from the pieces of spinal cord which he 

 identified as rudimentary motor nerves. 



Any possible doubt as to the correctness of this identification was 

 removed by Burrows (1911) repeating the work on the chick and 

 obtaining the specific staining reaction of neuro-fibrils in the 

 structures in question. These nerve -rudiments when kept alive 

 under the conditions mentioned were observed to increase rapidly 

 in length, the rate of growth being in one case as high as 56/* per 

 hour. The end of the rudiment (Fig. 57) was somewhat enlarged 

 and projected into fine protoplasmic tags which showed active 

 amoeboid movement. It is this amoeboid protoplasm at the free 

 end of the fibre which, in Harrison's belief, is the active agent in the 

 extension of the nerve-fibre. 



As to the method by which it is, in the actual body, guided along 

 the proper path to its destination, Harrison does not commit himself, 



but he appears to have a leaning 

 towards the view held by Ramon y 

 Cajal that it is mainly a matter of 

 chemiotaxy. 



In the words of their author 

 (1908) these " experiments place the 

 outgrowth theory of His upon the 

 firmest possible basis, — that of direct 

 observation. The attractive idea of 

 Hensen must be abandoned as un- 

 tenable." 



It should be added that the His 

 theory fits in very well with current 

 views in physiology and pathology — 

 in particular with the fashionable 

 neurone doctrine, according to which 

 the cellular units which compose 

 the nervous system are not in organic 

 continuity with one another. Ob- 

 viously this hypothesis and the 

 outgrowth hypothesis, according to 

 which the nerve-fibre is for a time 

 separated by a gap from its end- 

 organ, lend one another mutual 

 support. 1 



1 It must be borne in mind, however, that the histological basis of the neurone 

 theory is not universally admitted to be beyond suspicion. Its main foundation 

 consists of observations by the Golgi and similar methods of metallic impregnation. 

 In preparations made in this way single cellular units are frequently picked out 

 without the reaction taking place in neighbouring units arranged in series with 

 them. A ganglion-cell A with its axon and terminal branches stands out deep black 

 in the preparation, while the ganglion-cell B, next to it in the series, shows no 

 reaction. Such an observation obviously suggests discontinuity. 



The possible fallacy in these observations lies in the fact that the stain used is not 

 a true stain in the ordinary sense of the word hut merely a precipitation of metal upon 



Fig. 57. — View of outgrowing nerve- 

 fibre. Both figures are drawn from 

 the same live preparation, B twenty- 

 five minutes later than A. (After 

 Harrison, 1908.) 



