28 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



but not in others. In other cases the organisms were well 

 preserved and deeply stained. 



These experiments failed to demonstrate any significant 

 differences in the reactions of bacteria and mitochondria to 

 ether, alcohol and acetic acid. 



Reactions to physical agents. Mitochondria are known to 

 be sensitive to mechanical manipulation, desiccation and 

 shght variations in the physical environment. They 

 fragment readily. This sensitiveness of mitochondria is 

 usually spoken of as fragilitj^ It has been assumed that 

 bacteria do not possess such a fragiUty and that this char- 

 acteristic differentiates them from bacteria. GuilUermond 

 ('19), in particular, has stated that there are no bacteria 

 known that are as fragile as mitochondria. 



Experiments were devised in which the tissues of fetal, 

 young and old animals were smeared on glass slides, the 

 fihn dried, fixed and stained by various mitochondrial and 

 bacteriological methods. It was at once apparent in these 

 preparations that the fragiUty of mitochondria varies in the 

 different tissues, and with the age of the animals. In the 

 smears from adult animals mitochondria were seldom visible 

 either in the intact cells in the preparation or in the cyto- 

 plasm of the ruptured cells. In the tissue smears from 

 young animals, the mitochondria were present in the intact 

 cells, but seldom in the cytoplasm of ruptured cells. The 

 smears from fetal tissues invariably contained mitochondria 

 in great numbers in the fragments of ruptured cells as well 

 as in those still intact. 



These experiments seem to indicate that fragility of 

 mitochondria increases directly with the age of the animal. 



FragiUty in bacteria was not investigated experimentally. 

 Bacteriological Uterature contains ample evidence that 

 bacteria vary considerably in fragiUty. One microorgan- 

 ism, in particular, exhibits a degree of fragiUty properties 

 that, apparently is as great as that exhibited by mito- 



