108 THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 



dria are particularly numerous in the primordial muscle-cells, often being arranged 

 in chains, indicative, he thinks, of transformation into fibrils. Moreover, Romeis 

 (1913o, p. 10) has found a marked increase in mitochondria and transition forms 

 like those figured by Duesberg during the regeneration of muscle-cells in Triton. 



Too much weight should not be placed in the presence-and-absence argument. 

 There may be other reasons than fibril formation for oscillations in the amount 

 of mitochondria. Romeis attributes the increase in mitochondria to the more 

 embryonic condition of the cell. It is very possible that it may depend upon 

 increase in oxidations (see p. 82). Even should there be some association between 

 the decrease in the number of mitochondria and the formation of fibrils, it does 

 not follow that the mitochondria themselves change into them. The arrange- 

 ment of mitochondria in chains may simply be the outward and visible sign of the 

 formation of fibrils between them from non-mitochondrial precursors. 



The results thus far obtained with tissue cultures by Levi (1916c, p. 82) are dif- 

 ficult to reconcile with Duesberg's view, for Levi found that there was no relation- 

 ship whatever between mitochondria and the growth of fibrils in mesenchyme cells. 



We hold, not without some justification, that mitochondria are chemically 

 a combination of phospholipin with a small fraction of albumin. Now we are 

 asked to believe that, at a certain stage in the development of the embryo, fila- 

 mentous mitochondria, which to all our solubility tests and staining reactions are 

 alike and show no variability, in three different locahties become chemically trans- 

 formed into three different materials. In the myoblasts they are said to change 

 into myofibrils, which contain tyrosin; in the neuroblasts they are supposed to 

 change into neurofibrils, the chemical nature of which is unknown; and lastly, 

 they are also said to form connective-tissue fibrils, which yield collagen, a protein 

 devoid of tyrosin. But the mitochondria do not contain tyrosin (Cowdry, 1916a, 

 p. 427). Where, then, does it come from? Certainly not from the mitochondria. 

 Other more diflficult questions must be asked and answered before we can bring 

 ourselves to beheve in the chemical transformation of mitochondria into myofibrils. 



EPIDERMAL FIBRILS. 



Firket (1911, p. 537) has investigated the role of mitochondria in the differ- 

 entiation of epidermal fibrils in the egg tooth and feathers of chick embryos. His 

 material consists of three series of chick embryos. The first, from 8 to 15 days' 

 incubation, was fixed in Bouin's fluid; the second, from 6 to 21 days, in Flemming's 

 fluid as modified by Meves; and the third, embryos from 6§ days until hatching, in 

 Benda's fluid. Preparations from the first were stained in safranin or iron hema- 

 toxyhn, either alone or followed by a counterstain of rubin, eosin, and orange G. 

 Some sections from the second and third series were also treated in this fashion, 

 although the majority were stained with iron hematoxylin or crystal violet. 



Firket found (p. 540) a certain variability in the coloration of the fibrils after 

 fixing in Bouin's fluid and staining with iron hematoxyhn. The first ones to 

 appear were lighter colored and stained irregularly, whereas the completely differ- 

 entiated fibrils stained a darker uniform shade. He says (p. 544) that this "mon- 



