104 EMBRYOLOGY OK TIIK LoWKi; \ KIMTJ'.L'ATKS 



making their appearance from the pieces nf spinal cord which he 

 identified as rudimentary motor nerves. 



Any possible doubt as to the correctness of this identification \\as 

 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 aeti\e 

 uim>el>oid 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 

 ;. view of outgrowing nerve- outgrowth hypothesis, according to 



.,!,. Hotl, figUM are, drawn from whj h th nerve . fibre fe f()r a \ mu . 



tin- ^ame live preparation, B twenty- 



me minntei later ti,.,n A. (After separated by a gap from its end- 



Harris.,n. ! organ, lend one another mutual 



support. 1 



1 It must In- Imnif iii mind, h<>\\c\ -r, that the historical l.a.sjs of tin- ncuroiif 



:..t univiTsally admit ted i<> hr ln-ymnl suspicion. \\^ main foimdat ion 



| tin- (iol^i and .similar met hods of metallic impregnation. 



In preparations mad.- in tln> \\ay single e.dlular units aiv l'n-|in'iitly picked uit 

 witliont the i-M-tin taking pla-,- in n< i^lil.nirin^ unit> arrant' 1 '! in ^erie> \\ith 

 them. A ^an^lion-eell A \villi ilsa\<n and ti-rminal luanehes stands out deep lilack 



ill the pp-paiatioll, while tin- 11 li, ln-\t hi it ill the sellev, sl,o\\s Ho 



11. Sueh .1 ' - diM-ont innity. 



'!'}). |i..s-ilil- fallacy in I ID lh'- I'a.-l that the stain used is not 



in the old "I tin- \\oid I, ut merely a precipitation of metal upon 



