98 THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 



tioned. Faure-Fremiet (1910a, p. 527) has also forwarded strong evidence that 

 the mitochondria in certain infusoria multiply in this way. The multiphcation of 

 mitochondria in mammaUan tissues is a phenomenon exceedingly difficult to demon- 

 strate. As a possible source of error it should be borne in mind that the most com- 

 mon reaction on the part of filamentous mitochondria to unusual conditions is to 

 fragment — that is to say, to multiply by transverse division. This is a very defi- 

 nite reaction to injury, by no means a normal method of multiplication. Again, 

 in sectioned material it is impossible to say with certainty whether the appear- 

 ances observed mean an actual multiplication by division or simply an approxima- 

 tion of originally separate mitochondria, for mitochondria everywhere show a 

 tendency to clump together which is markedly enhanced if the tissue is in any way 

 injured. The only way to obtain definite information on the subject is to study 

 a process, rather than a process killed at a certain point, as in fixed material, 

 or even a perverted process, as in living cells teased out and growing in isotonic 



media. 



As against the doctrine of mitochondrial continuity, we have certain cells in 

 which the mitochondria appear to go into solution. I have found that this is the 

 case in the chromophile cells of the nervous system (Cowdry, 1916/), p. 41). There 

 have been, so far as I am aware, but few observations (Chambers, 1915^ p. 291) of 

 a de novo origin of mitochondria in the cytoplasm, but to my mind the burden of 

 proof is not that it occurs, but rather that it does not take place. It would surely 

 be arbitrary to assert that the phosphatid albumin complex (of which mitochondria 

 consist) always occurs in certain aggregates of definite size which are visible with 

 our present powers of the microscope and that these aggregates multiply by divi- 

 sion like independent organisms. In this connection we can only speculate, but it 

 certainly seems much more likely that the phosphatid may be deposited free in the 

 cytoplasm or upon an albuminous matrix, and that it subsequently grows by 

 accretion. It is quite possible that when it attains a certain size or shape it divides 

 for purely physical reasons; but in the absence of definite proof, one way or the 

 other, it seems to me highly j^robable that mitochondria are continually arising 

 in the cytoplasm de novo, and furthermore that perhaps this is the most important 

 method of multiplication. 



It is, however, incumbent upon those who believe that the mitochondria are 

 carriers of heredity to demonstrate their continuity; and, further, the origin of 

 mitochondria de novo, if it does take place, would be very difficult to reconcile with 

 the view that they transmit hereditary traits. 



The observations of Beckwith (1914, p. 216) on the eggs of Hydradinea echi- 

 nata are of particular interest in this connection. She found that the cytojilasmic 

 contents of the fertilized eggs on centrifuging separated into three layers— a layer 

 of oil, a clear zone, and a layer containing mitochondria and vitellus. The first 

 plane of segmentation cut these layers in various directions, resulting in planulas of 

 very diff'erent appearance. Beckwith separated the two first blastomeres of cen- 

 trifuged eggs and allowed them to develojx Some of these formed planulas, which, 



