SYMBIONTICISM IN RELATION TO HEREDITY 127 



pecially differentiated. Certainly, we have a differentia- 

 tion of the cell as a whole, but this differentiation goes 

 hand in hand with mitochondrial differentiation, and must 

 be more or less dependent upon it. The evolutionary origin 

 of the specialized cell depended upon the microsymbiont. 

 The herditary or ontogenetic origin of the specialized cell 

 must be dependent upon the presence of the same factor 

 (mitochondria). The question then resolves itseK into the 

 determination of the nature of the modifications which the 

 microsymbiont undergoes in absolute symbiosis. 



We have very Httle direct evidence on which we can form- 

 ulate an h\^othesis as to the intimate nature of the relation- 

 ships of microsymbionts to the host cell and to the germ- 

 plasm. Certain general features of bacteria on the one 

 hand, and mitochondria on the other, are suggestive, and 

 at least furnish a tentative or working hypotheses. 



While all bacteria do not contain granules which might 

 be interpreted as chromatin, a great many do. It is a 

 rather singular coincidence that mitochondria have never 

 been observed to contain any granules that could be inter- 

 preted as chromatin. One might expect that at least some 

 of the microsymbionts originally (in the free state) contained 

 visible chromatin. We mentioned above the possibihty 

 that chromatin material from the microsymbionts might 

 supply "new" genes to the chromatin of the germplasm. 

 It appears possible for a part or all of the chromatin of a 

 microsymbiont to be given up to the nucleus of the host 

 cell and germplasm when symbiosis develops, leaving the 

 remains of the microsymbiont in the cytoplasm somewhat 

 in the nature of a plast. The part of the chromatin re- 

 moved from the microsymbiont may not be essential for 

 its growth and reproduction, but it is conceivable that the 

 specific activity of the mitochondria is maintained only 

 under the influence of this chromatin. This influence 

 may be exerted 'at long range' from the nucleus. It 



