SYMBIONTICISM AND ORGANIC EVOLUTION 135 



the chromatin increased, and perhaps became morpholog- 

 ically modified with each fusion, ultimately forming a defi- 

 nite body — the nucleus — surrounded by the cytoplasm. 



Obviously, the formation of a defioiite nucleated cell did 

 not produce a structure that became absolutely fixed and 

 incapable of further prototactic activities. Subsequent 

 "fusions" resulted in myriads of different kinds or species 

 of one-celled organisms some of which may be in existence 

 today. We also recognize in these plants and animals the 

 'partially fused' microorganisms (mitochondria) that appear 

 to have been and undoubtedly still are the fundamental 

 factor in the production of new forms. 



It is well to consider again the modifications that bac- 

 teria exhibit when they become symbiotically associated 

 with a cell. In the discussion of the early stages of evolu- 

 tion, we assumed that the bacteria "fused" to ultimately 

 produce a nucleated cell. In such fusions, the identity of 

 the original bacteria may have been completely lost. In 

 the mitochondrial microorganism, a certain identity is 

 retained by the microsymbiont, the mitochondria being 

 present as definite bodies in the cytoplasm of the host cell. 

 It must again be emphasized that microsymbionts may 

 undergo even radical modifications. A number of bac- 

 terial symbionts retain their independence to the extent 

 that they are readily cultivated in ordinary culture media. 

 Others appear to develop fragiUty, and, perhaps become 

 modified in other ways making them difficult to stain and 

 to cultivate in artificial media. We have previously dis- 

 cussed the behavior of mitochondria, and mentioned the 

 phenomenon in which they seemed to "dissolve" in the 

 cytoplasm. We may again direct attention to the obser- 

 vations of Chambers, in which he apparently saw the mito- 

 chondria go into solution and again reappear as bodies in 

 the cytoplasm. It appears probable that in some instances 

 the microsymbiont completely fuses with the host cell. It 



