134 R> W ' GLASER. 



6. In adult insects the symbionts are sometimes found free in 

 the lymph, but in most species are found enclosed in very definite, 

 modified cells known as mycetocytes or bacteriocytes. 



7. In some species two or more mycetocytes fuse and become 

 concentrated forming a very definite organ known as the myce- 

 tome. Some insects are monosymbiotic, that is, they harbor only 

 one species of symbiont ; others are disymbiotic or trisymbiotic. 



9. When the host is monosymbiotic the symbionts may be found 

 in mycetocytes or may be concentrated in a mycetome. 



10. When the host is disymbiotic one species of symbiont may 

 be found in mycetocytes and the other in a mycetome, or both 

 symbionts in separate mycetomes or both symbionts in separate 

 feebly connected mycetomes or lastly both species may be in one 

 mycetome. 



11. When the host is trisymbiotic we may have two species of 

 symbionts in one mycetome and the third in mycetocytes or the 

 three symbionts may be in one mycetome. 



12. The host cells show specific changes, but no injury. They 

 usually enlarge. If the symbionts are small the cytoplasm has 

 the appearance of being finely reticulate ; when large the cyto- 

 plasm becomes coarsely reticulate. The nuclei of the host cells 

 are sometimes round ; at other times curved or even arborescent. 

 The organisms never invade the nuclei. In spite of deformation 

 of the cytoplasm, the nucleus retains its ability to divide. Some- 

 times one finds nuclear and cytoplasmic degeneration, but this is 

 an exception and probably due to some, secondary parasite or to 

 disease. 



13. Most of the mycetocytes are enormous in size when com- 

 pared with the remaining cells of the animal. 



14. The systematic position of the symbionts is not always 

 clear. In the case of the. Blattids they are undoubtedly bacteria ; 

 in the case of some aphids undoubtedly yeasts, and in the other 

 cases yeast-like organisms and probably higher fungi. No proto- 

 zoan intracellular symbionts of insects are known. 



15. The symbionts from a small number of host species have 

 been cultivated upon artificial media, but in most cases the arti- 

 ficial cultivation experiments of symbionts have so far met with 

 unsurmountable difficulties. This may be due to the fact that 



