408 E. V. COWDRY 



respectively. In both cases, drawings have only been made from 

 sections of the neural tube on the right side opposite the sixth 

 somite, and the degree of specialization of the embryos from 

 which the figures on plate 3 were drawn corresponds severally 

 with that of those represented on plate 4, figure 15 a to figure 

 15 b, and so on. The preparations show that in progressive 

 stages in the differentiation of the cells of the neural tube there 

 is no decrease in the amount of mitochondria parallel to the 

 increase in neurofibrillar material. 



DISCUSSION 



1 . Foundation of the theory that the neurofibrils are developed from 



mitochondria 



This hypothesis is based to some extent upon the following 

 statements, which have been advanced, in whole or in part, by 

 the investigators named. 



a. That the neurofibrils increase in amount as the mitochon- 

 dria decrease until finally the adult condition is attained in which 

 the neurofibrils are completely differentiated and the mitochon- 

 dria absent. (Duesberg '10, p. 512; Hoven '10, p. 478; and 

 Meves '10 a, p. 655). 



b. That microchemical transitions exist between mitochondria 

 and neurofibrils, since the primitive neurofibrils may first be 

 stained by mitochondrial methods, then by both mitochondrial 

 and neurofibrillar methods and, finally, by the various neurofi- 

 brillar methods of technique alone (Meves '08, p. 838; Hoven 

 '10, p. 478, etc.). 



c. That morphological transitions also exist between mito- 

 chondria and neurofibrils: according to Meves ('08, p. 838) 

 chains of mitochondria are transformed into neurofibrils; ac- 

 cording to Hoven ('10, p. 475) the mitochondria form a reticulum 

 from which the neurofibrils are differentiated. 



d. That the development of the myofibrils, connecti\^e 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 (Benda '99; Meves 

 '07; Duesberg '10; Meves '10; Firket '11 and Duesberg '12). 



