412 E. V. COWDRY 



o\'er these properties apparently differ in no wise from those of 

 mitochondria in the neural tube before the formation of neuro- 

 fibrils or from the mitochondria in other embryonic cells. 



It is evident therefore that the facts do not justify the state- 

 ment that microchemical transitions exist between mitochondria 

 and neurofibrils. 



(3) With respect to the evidence for morphological transitions 

 between mitochondria and neurofibrils I would state that I have 

 failed to confirm j\Ieves' contention that chains of mitochondria 

 are transformed into neurofibrils. Mitochondria are sometimes 

 oriented end to end and one msiy often observe very long fila- 

 mentous mitochondria, like those represented in figure 4, for 

 instance. It is howe\'er a very far cry from either a linear ar- 

 rangement of mitochondria or from long filamentous mitochon- 

 dria to neurofibrils. This is manifested, among other things, b}' 

 the fact, already mentioned, that there is nothing peculiarly dis- 

 tinctive about the morphology or the arrangement of mitochon- 

 dr'a in the cells of the neural tube during neurofibrillar forma- 

 tion: they are alike indistinguishable, on the basis of their 

 morphology and distribution, from the mitochondria in the neural 

 tube in stages prior to the differentiation of neurofibrils, and 

 from the mitochondria occurring in other embrj^onic tissues both 

 before and contemporaneous with the de\'elopment of neuro- 

 fibrils. So that on the ground of morphology and cytoplasmic 

 arrangement of mitochondria there is just as much evidence for 

 the formation of neurofibrils in structures derived from meso- 

 derm and endoderm as there is in the case of the neural tube. 



Since Hoven's own figures do not show a reticulum, but rather 

 an interlacing of independent mitochondrial filaments, and since 

 I have been unable to discover a reticulum composed of mito- 

 chondria in an}^ of my preparations, the presumption is war- 

 ranted that the mitochondria do not lose their individuality by 

 coalescing to form a network. 



The conclusion is likewise justified that the facts, so far as 

 we know them do not support the statement that morphological 

 transitions occur between mitochondria and neurofibrils. 



