112 THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 



Finally, the neurofibrils are said to enter upon a third phase in their history 

 characterized by the loss of their affinities for mitochondrial dyes. I have never- 

 theless failed to find any conclusive evidence that the neurofibrils change their 

 chemical properties after their first formation. My failure may be due to the 

 unstandardized condition of the neurofibrillar methods of technique which still 

 prevails. In any case the burden of supplying the evidence rests with those who 

 make the statement. If the neurofibrils are formed by a chemical transformation 

 of mitochondrial substance into neurofibrillar material, one would expect to find 

 variations in the effects of fixation and in the staining properties of mitochondria 

 during their formation. I have shown that the exact converse obtains. Both 

 the solubility of mitochondria in acetic acid and the staining reactions of mito- 

 chondria in the cells of the neural tube in which neurofibrils are being actively 

 formed remain remarkably uniform and constant. Moreover, these properties 

 apparently differ in no wise from those of mitochondria in the neural tube before 

 the formation of neurofibrils or from the mitochondria in other embryonic cells. 

 It is evident, therefore, that the facts do not justify the statement that microchem- 

 ical transitions exist between mitochondria and neurofibrils. 



(c) With respect to the evidence for morphological transitions I would state 

 that I have failed to confirm Meves's contention that chains of mitochondria are 

 transformed into neurofibrils. Mitochondria are sometimes oriented end to end, 

 and one may often observe very long filamentous forms. It is a very far cry from 

 a hnear arrangement of mitochondria or from long filamentous mitochondi'ia to 

 neurofibrils. This is manifested, among other things, by the fact already men- 

 tioned, that there is nothing pecuharly distinctive about the morphology or the 

 arrangement of mitochondria in the cells of the neural tube during neurofibrillar 

 formation; they are ahke indistinguishable, on the basis of their morphology and 

 distribution, from the mitochondria in the cells of the neural tube in stages prior 

 to the differentiation of neurofibrils, and from the mitochondria occurring in other 

 embryonic tissues both before, contemporaneous with, and after the development 

 of neurofibrils. Therefore, on the ground of the shape and cytoplasmic arrange- 

 ment of mitochondria, there is just as much evidence for the formation of neuro- 

 fibrils in structures derived from mesoderm and endoderm as there is in the case of 

 the neural tube. 



(d) The value of the argument from analogy has already been made plain 

 by the preceding discussion of the development of other fibrillar structures. 



It seems clear that the neurofibrils are not formed by a transformation of 

 mitochondria. They are elusive structures. They can not be seen in the living 

 cell, or with the aid of vital dyes; neither can they be dissected out (Kite^. There 

 is every reason to believe that they do not occur in the living cell in the form in 

 which we see them in our silver preparations. They are formed of material quite 

 different from the Nissl substance, mitochondria, or canalicular apparatus, though 

 we neither know what it is nor the factors concerned. 



'Dr. Kite, personal communication. 



