THE MITOCHONDRTAL CONSTITUENTS OF PROTOPLASM. 113 



It has been claimed that many other fibrillar structures are formed from mito- 

 chondria. Saguchi (1913, p. 239), for example, has made a careful study of the 

 development of the tonojibrils and intracellular structures of Eberth in the epidermal 

 cells of batrachian larvae and concludes that they arise through a chemical trans- 

 formation of mitochondria. His figures show a very definite association of the 

 mitochondria with the fibrils and all morphological gradations between the two 

 are represented. His evidence, however, for a chemical change (p. 241) is not 

 conclusive, for it does not exclude the possibility that the substances making up 

 the fibrils arise elsewhere and are deposited within the mitochondrial filaments 

 without change of the mitochondrial material. The fibrils of Herxheimer are 

 considered by Favre and Regaud (1910, p. 1138) to be true mitochondria. Meves 

 (1907a, p. 403) has suggested that the neuroglia Jibri Is are hkewise developed from 

 mitochondria, but has forwarded no evidence in substantiation. 



In concluding this discussion of the histogenesis of fibrils it may be remarked 

 that it is somewhat illogical to suppose that substances of such diverse chemical 

 constitution are all formed through the transformation of a single substance, a 

 phospholipin, combined perhaps with a small fraction of albumin. The coUagenic 

 fibrils on boiling yield gelatin, a protein devoid of tyrosin; the neurofibrils are of 

 unknown composition, and we even doubt their existence in the living cell; certain 

 of the epidermal fibrils contain a keratin-like material, and our chemical acquaint- 

 ance with the neuroglia fibrils is of the slightest. It seems far more likely that the 

 fibrils are, from the outset, different from one another— that they are formed from 

 different materials rather than from the same material. They may be formed 

 through a condensation of substances in the cytoplasm, either in the form of minute, 

 perhaps ultramicroscopic, particles which tend to be arranged in rows following 

 lines of stress or strain in the cell and which naturally fuse together, end to end, in 

 accordance with the law of least surfaces; or, it is entirely conceivable that there 

 may be only a single center of condensation which grows and enlarges by the addi- 

 tion of more material by accretion, something like a crystal. In connection with 

 the first of these alternatives we know that Unes of stress do exist in living proto- 

 plasm, for the fibrils are deposited at a time in development when growth activi- 

 ties are greatly pronounced, when cellular migrations are common, and changes 

 in the form of various parts of the body quite frequent. The fully formed fibrils 

 undoubtedly correspond, in position, to fines of stress and strain. We know that 

 certain cytoplasmic elements are subject to orientation along such lines. I refer, 

 particularly, to the arrangement of material in lines about the centrosomes, to the 

 arrangement of granules in the rootlets of ciUa?, to the characteristic deposition 

 of material in bone, etc. 



PLANT PLASTIDS. 



By far the most convincing evidence in favor of a participation of mitochondria 

 in histogenesis, through an actual chemical transformation of their substance, is 

 to be found in the botanical literature. In fact, these newer methods of mito- 

 chondrial technique strike at very important problems, for they have a definite 

 bearing upon the origin of all plastids. 



