CYTOPLASMIC CONSTITUENTS OF NERVE CELLS 393 



^larcora ('11, p. 952), so far as I have been able to determine, 

 was the first investigator to criticise this conclusion and to em- 

 phasize the following differences between the chondriosomes and 

 the primitive neurofibrils: (1) that the chondriokontes are short, 

 thick, wavy filaments, never being continuous with the few chon- 

 driosomes in the axis cylinder; whereas the neurofibrils are fine, 

 long, straight threads arranged near the greater process of the 

 neuroblast and distinctly continuous with the numerous neuro- 

 fibrils in the axis cylinder; and (2) that it is not possible to stain 

 distinctively the neurofibrils with the mitochondrial dyes. 



Furthermore, Duesberg ('12, p. 745) returns the verdict 'non 

 proven' in the case of the neurofibrils, but maintains that he 

 has incontestable^ demonstrated that myofibrils arise in like 

 fashion from mitochondria. 



The regional differentiation of neurofibrils in the chick will 

 be considered under three headings: 



(a) The stage of development at which the first neurofibrils 

 appear. Besta ('04, quoted from Collin '06, p. 238) gives the 

 degree of differentiation as that of sixty-five hours incubation; 

 Cajal ('07, p. 178) fifty-six to sixty hours; Gerini ('08, p. 182) 

 forty hours, and Hoven ('10, p. 441) 44 somites and seventy-six 

 hours incubation. Other investigators working prior to these 

 with more imperfect methods give the date even later. 



(b) Type of cell in which neurofibrils first develop. Besta ('04, 

 quoted from Collin '06, p. 238) and Gerini ('08, p. 182) conclude 

 that the neurofibrils in the chick are formed in the bipolar cells 

 of the outer layer of the neural tube; Cajal ('07, p. 178) in the 

 apolar cells bordering the ventricle (i.e., in preneuroblastic cells); 

 and Meves ('07, p. 403) and Hoven ('10, p. 478) assert that they 

 are first differentiated in the cytoplasm of the neuroblasts. 



(c) Region of the cytoplasm in which neurofibrils originate. 

 Investigators are apparently agreed that the neurofibrils in the 

 chick are first formed in a definite, restricted zone of the cyto- 

 plasm; for Besta ('04, quoted from CoUin '06) claims that they 

 are formed in the protoplasmic substance about the nuclei; 

 Cajal ('07, p. 178) that they are differentiated in a network 

 which arises in a fibrillogenous area in the distal portion of the 



