Evidence from Somatic His to genesis 39 



as to what "hereditary substance" is. 



The results and conclusions thus far reached are on the 

 whole so diverse, often so conflicting, that any attempt at 

 a general review of them would be useless for this discussion. 

 However, certain of the results, actual or claimed, are im- 

 portant. For example, at one extreme it is contended that 

 the mitochondria are the immediate precursors of the most 

 distinctive elements of ah 1 classes of adult tissues. Thus 

 Meves : "All these differentiations (of embryonic cells into 

 tissue cells) however heterogeneous they may be, arise 

 through the metamorphosis of one and the same elementary 

 constituent of the plasma, the chondriosomes. The chondri- 

 osomes are the material substratum lying at the basis of the 

 processes of differentiation, which become the specific sub- 

 stances of the different tissues." As an extension of this 

 view we learn from Lewis and Lewis and other reviewers 

 that the following tissues are reported on the authority of 

 a long list of workers to be produced by the mitochondria: 

 fibrillae, myofibrillae, fibrillae of epithelial cells, corneous sub- 

 stance, secretory granules, fat, leuco-, chloro- and chromo- 

 plasts, the test substance in foraminifera, and various other 

 tissue elements. 



But several investigators, notably E. V. Cowdry, have 

 shown the inconclusiveness of the evidence on which the con- 

 tention is based that neurofibrils originate from mitochon- 

 dria. "There is no evidence," Cowdry says, "that mito- 

 chondria are transformed into neurofibrils. . . . The mi- 

 tochondria do not show, either by a variation in their 

 morphology, staining reactions, or in any other fashion, 

 . . . indications of being transformed into material of dif- 

 ferent chemical composition." 7 Furthermore, lie shows that 

 the mitochondria do not diminish in quantity in any way 

 commensurate with the increase of neurofibrils, as the neu- 

 roblasts transform into ganglionic cells. Eminently worthy 

 of note, as bearing on our contention made some pages 



