126 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



in case of the annelid Choetopterus, in which the residual substance 

 is more specifically described as constituted of fine granules or 

 microsomes visible in life before and after their discharge from 

 the germinal vesicle. 



Regarding the role played by chromidia in cellular dif- 

 ferentiation, we must admit that we know very little, if 

 anything. They have been described by a large number of 

 cytologists and have been variously interpreted. The 

 relationship of mitochondria to cellular differentiation 

 assumes a new significance when we bear in mind their 

 bacterial nature and their relationship to Symbionticism. 

 We have mentioned in another place that bacteria in nature 

 are known to produce the same enzymes and metabolic 

 products that are formed by the tissues of plants and 

 animals. Bacteria have been shown to develop intimate 

 symbiotic relationships with plants and animals. In these 

 symbioses it has been observed that ''new" cells develop 

 (modifications of preexisting cells) to harbor the symbionts. 

 Examples have been given in which new organs developed 

 in response to the presence of microsymbionts. Are these 

 observations to be interpreted as biological digressions 

 without any fundamental significance? Must we discard 

 these apparent facts without attempting to correlate them 

 with other cytological observations? In the result of the 

 researches of a large number of investigators, the onto- 

 genetic differentiations of cells have been associated with 

 mitochondria. These researches have not been accepted 

 by most cytologists, apparently because of an assumed 

 incompatible chemical constitution of mitochondria. We 

 are forced to the conclusion that mitochondria constitute a 

 fundamental factor in cellular differentiation. 



If mitochondria, as we have claimed, are the "specific" 

 part of the cell, that is, the part that is concerned with the 

 special function of a differentiated cell, then in cellular 

 differentiation it must be the mitochondria that are es- 



