128 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



may be that this substantiates Schaxel's suggestion that 

 the ''chromidia" from the nucleus may function in some 

 manner in connection with the mitochondria in histogenesis. 



A certain amount of experimental evidence indicates 

 that the mitochondria undergo modifications during the 

 life of the host. This was suggested by the fragihty ex- 

 periments, as well as by the growth reactions of cultured 

 mitochondria. While these experiments did not demon- 

 strate any variations in the chromatin content (chromatin 

 granules were not visible in any cultured mitochondria), 

 they suggest an alteration in function accompanjdng a 

 modification in metaboUsm, as indicated by a change in 

 food requirements. These results were thought to indi- 

 cate that the mitochondria in the embryo and fetus possess 

 no specific functions, but that these develop or differentiate 

 later in the Ufe of the organism. Obviously, specific tissue 

 and organ function varies greatly in the different tissues of 

 the developing animal. Some functions develop in early 

 fetal Ufe, others certainly not until after birth. Temporary 

 functions (hemopoiesis) are present in the hver before birth 

 and are continued for a short time thereafter. 



In a large number of s3niibioses, the independent prop- 

 erties of the microsymbiont apparently are retained. In 

 some cases, although the microsymbiont may occupy an 

 intracellular position, the bacterial nature of the micro- 

 symbiont is readily detected. While we have no way of 

 knowing, it appears reasonable to presume that in such 

 cases of symbioses the bacterial chromatin is retained by 

 the microsymbiont. It also appears possible that in some 

 cases of symbiosis, the microsymbiont completely fuses 

 with the host cell so that it no longer retains a morphologic 

 identity. 



The nature of the exact responses of microsymbionts 

 in symbiosis, remains to be determined. One thing ap- 

 pears certain, namely, that if the evolutionary origin of a 



