MORPHOLOGY OF EYE MUSCLE NERVES 29 



matic connections which form the nerve paths as secondary in 

 origin. He has, however, failed to make a detailed study of 

 the genesis of those plasmodesmata which, according to his view, 

 are later transformed into the somatic motor nerve aniagen. 

 Furthermore, one searches in vain through Held's admirable 

 monograph for a convincing proof of the assertion (p. 274) that 

 ''the growth of the nerve substance is correlated with a resorp- 

 tion of the plasmatic path so that the latter passes over into the 

 former and is utilized in its formation." Similarly unproven is 

 the dogmatic assertion (p. 280) that the 'plasmatische Ausfliisse' 

 of Dohrn ('88) are really not such. Dohrn's characterization 

 of the first protoplasmic connections of tube and myotome as 

 'plasmatische Ausfliisse' seems admirable. That they quickly 

 lose this undifferentiated protoplasmic character may readily be 

 granted, so that Held's assertion is correct except for the very 

 first stages in the appearance of the protoplasmic bridges. 



A number of important questions are involved in the issue 

 raised by Held and Paton: (1) Are protoplasmic connections 

 between myotome and tube primary or secondary? (2) What 

 cells participate in their formation? (3) Have these protoplasmic 

 paths a genetic relation to the neurofibrils? That is, do the 

 same cells which form the plasmodesms also differentiate the 

 neurofibrillae? The writer ('03). concluded that the protoplasmic 

 connections are secondary; that they are formed exclusively by 

 processes of medullary neuroblasts; that within them the neuro- 

 fibrils* are differentiated. Paton ('07) was not able to determine 

 whether they are primary or secondary; suggests (p. 560) that 

 mesenchymatous cells may participate in their production; and 

 concludes that the neurofibrils have no genetic relation with the 

 protoplasm, of the bridges, but arise independently within the 

 myotome and grow centripetally into the tube along the bridges. 

 On the other hand Held ('06, '09) regards the plasmodesms as 

 secondary in origin; but holds that they are formed by the fusion 

 of cells of chorda, tube and myotome; maintains that they have 

 no genetic relations to the neurofibrils which grow into them 

 secondarily from medullary neuroblasts, for which the paths pro- 

 vide a protoplasmic covering. The great theoretical importance 



