124 ZOOLOGY 



is generally accomplished by an elongation of the main axis. 

 There appears to be a consensus of opinion, hov/ever, that the 

 mitochondria are capable of storing material within their sub- 

 stance. Under these latter circumstances enlargement in the other 

 two axes takes place, the mitochondria becoming fatter instead 

 of longer, as in the case of true growth. 



The generally-accepted criterion of activity of the mitochondria 

 is growth. This may either take the form of elongation without 

 division, in which case they may assume very If ng filamentous 

 forms, or else division may take place as rapidly as the elongation, 

 so that a great multiplication of mitochondria takes place. This 

 is frequently found as a recurrent phase in gland cells. Another 

 form of activity is probably shown by the other form of enlarge- 

 ment, namely by a sweDing of the filaments or granules. The 

 actual nature of this action is not known. It is found most 

 frequently in oogenesis and spermatogenesis. 



There are other cases, to which reference will be made later, 

 in which a definite activity apparently takes place, but is asso- 

 ciated with neither of these changes in form, but rather by a 

 diminution in size, and even total reduction. 



One of the problems of the earlier workers was the actual 

 standing of the mitochondria as cell organs. Many workers 

 were inclined to believe that they were really symbiotic bacteria, 

 since many of their reactions are the same, and that such 

 differences as exist are due to the fact that they are quite definitely 

 modified for the symbiotic mode of life, and that this has probably 

 been going on for a very long time. The question is not entirely 

 one ol technique, as evidence that mitochondria arise de novo 

 would militate strongly against the symbiosis theory. On the 

 other hand, it must be remembered that it is extremely difficult 

 to prove such an origin, as it may be that the mitochondria are 

 not really absent, but are merely in a non-staining phase in the 

 cell, and that their apparent production de novo is simply a return 

 to a stainable phase. The weight of the evidence is, however, on 

 the side of those who maintain that the mitochondria are definite 

 cell organs of an equal standing at least with the Golgi ai3paratus, 

 with which they are classed as protoplasmic inclusions. 



