110 THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 



Firket's evidence is not conclusive, because he has not demonstrated the 

 existence of a complete series of transitional stages showino- tlie loss in the jiroiier- 

 ties of the fibrils. He has, however, gone further than Aleves did in connection 

 with the collagenic fibrils, or Duesberg in the case of the myofibrils, because he 

 succeeded in demonstrating a variability in the staining reaction of the first fibrils 

 to appear. 



NEUROFIBRH.S. 



Many iinestigators have touched on the question whether mitochondria play 

 a part in the differentiation of neurofibrils, but Hoven (lOlOo, p. 427) in partic- 

 ular, working with chick embryos, has furnished the most complete evidence in 

 favor of the view that mitochondria are actually transformed into them. 



This interesting conception has been supiwrted by Meves, who originally 

 enunciated it (1907rt, p. 403), as well as by G. Arnold (1912fl, p. 288), and several 

 others to be mentioned subsequently; it has been rejected by Marcora (1911, p. 

 952), Levi (1911, p. 180), and Gurewitsch (1913, p. 126), while Duesberg (1912, 

 ]). 745) has assumed a non-committal attitude with regard to it. 



It is based upon the following statements : 



(a) That the neurofibrils increase in amount as the mitochondria decrease, until 

 finally the adult condition is attained in which the neurofibrils are completely differ- 

 entiated and the mitochondria absent (Hoven, 1910o, p. 478; Meves, 19106, p. 655). 



(6) That microchemical transitions exist between mitochondria and fibrils, 

 since the primitive neurofibrils may first be stained by mitochondrial methods, 

 then by both mitochondrial and neurofibrillar methods, and finally by the various 

 neurofibrillar methods of technique alone (Meves, 1908, p. 838; Hoven, 1910a, 

 p. 478, etc.). 



(f) That morphological transitions also exist between mitochondria and 

 neurofibrils; according to Meves (1908, p. 838), chains of mitochondria are changed 

 into neurofibrils; according to Hoven (1910a, p. 475), the mitochondria form a 

 reticulum from which the neurofibrils are differentiated. 



{d) That tlie development of myofibrils, connective-tissue fibrils, and the 

 fibrils in epithelial cells support this theory, since they, in a similar fashion, are 

 developed from mitochondria. This constitutes the argument from analogy (Meves, 

 1907fl, p. 403; Duesberg, 1910, p. 613; Meves, 1910a, p. 162; Firket, 1911, p. 545; 

 and Duesberg, 1912, p. 759). 



The bearing of my own observations (1914^/) upon the statements upon which 

 the theory of the mitochondrial origin of the neurofibrils rests is as follows : 



(a) My own findings are utterly at variance with the first argument, for I 

 can discover no decrease in the amount of mitochondria running parallel to the 

 formation of neurofibrils. Moreover, the statement that they are absent in the 

 adult condition is wholly unwarranted in view of the fact that several investigators 

 had already unquestionably seen mitochondria in adult nerve-cells (p. 101). 



[b) The second statement postulates the existence of three distinct phases 

 in the development of the neurofibril, each of which is characterized by certain 

 microchemical properties. In the first stage the primitive neurofibrils may, it is 



