ii NEKVE DEVELOPMENT 117 



growth of the motor trunk from the spinal cord, but differing from it 

 in the essential feature that the outgrowth is simply protoplasmic 

 and not fibrillar, have been enunciated by some modern workers such 

 as Dohrn and Held. Dohrn (1888) describes the motor nerve-rudi- 

 ment as arising by a " plasmatic outflow from the neural tube " but 

 Paton later on finds that at the stage referred to by Dohrn the 

 nerve -rudiment is already continuous at its outer end with the 

 protoplasm of the myotome. 



Held (1909) also regards the motor trunk as arising by outgrowth 

 from the spinal cord at a time when the myotome is still com- 

 paratively close to it. It has to be borne in mind in interpreting 

 such sections as Held figures that there is more liability to error in 

 demonstrating the absence of continuity than in demonstrating its 

 presence, owing to the extremely fragile character of the nerve- 

 trunks during early stages in development and their consequent 

 liability to rupture during the ordinary processes of preparation 

 which precede section-cutting. 



It is sometimes said that the difficulty attaching to the His view 

 involved in the idea of the .nerve-fibre tracking down its own 

 particular end-organ disappears if the view is taken that the out- 

 growth takes place, at a stage so early as that indicated by Dohrn 

 and Held. But as 1 a matter of fact this involves, as indicated, a 

 distinct departure from the view enunciated by His according to 

 which not merely undifferentiated protoplasm but definite fibrillated 

 trunks grow out from the spinal cord. Further if, as Held believes, 

 the individual fibrils grow out in the substance of the protoplasmic 

 outgrowth each one has still to seek out the particular portion of the 

 myotome which will eventually be converted into its own proper 

 muscle-cell— a view which, looking to the comparatively undiffer- 

 entiated condition of the myotome cells at these early stages, is even 

 more difficult to comprehend physiologically than the outgrowth 

 towards a specialized muscle. 



The embryological evidence upon which the His view rests is 

 seen, when submitted to critical examination, to be unconvincing. 

 The same is the case with the observational evidence upon which 

 the Balfour view rests. The nuclei and cell-bodies which commonly 

 give a multicellular appearance to the nerve -rudiment are quite 

 reasonably interpretable as sheath -cells, i.e. mesenchyme elements 

 which have collected round and it may be migrated into the, at 

 first noncelmlar, nerve-trunk. 



In Lepidosiren, with its coarse and heavily yolk-laden mesen- 

 chyme, it is comparatively easy to distinguish such elements from 

 the actual nerve-trunk embedded in them, but in most Vertebrates 

 this criterion is not available and there is no certain means of 

 distinguishing in ordinary microscopic preparations the protoplasm 

 of the nerve-trunk from that of the sheath-cells. 



The primitive protoplasmic bridge described in 1902 for 

 Lepidosiren as representing the motor trunk at a time, when 



