198 THE ACTION OF THE LIVING CELL 



taining cytost an animal may suffer an involvement 

 of the lungs varying in severity from nothing to a true 

 pneumonia. This is especially interesting because re- 

 spiratory involvements of one sort or another are fre- 

 quently responsible for a high mortality amongst small 

 mammals, particularly rabbits and guinea pigs. Such 

 troubles are usually attributed to bacterial infections, 

 although as the writer has pointed out elsewhere 

 (Turck, 1919), it appears that the bacterial invasion 

 usually follows the congestion of the lungs. 



The fact that animals may suffer from the toxic ef- 

 fects of homologous cytost in the surrounding air sug- 

 gests that this is an important factor in producing the 

 effects of overcrowding. Various experiments which 

 we shall now consider substantiate this conclusion. 



When the dust which accumulates about animal 

 pens or cages was collected, converted into a sterile ex- 

 tract, and injected into homologous animals, it was 

 found that the animals died rapidly, in some instances 

 almost immediately, the time of death depending upon 

 the quantity of the extract injected. A similar finding 

 was obtained when concentrated extracts of the hair 

 and epithelial tissues were substituted for the dust ex- 

 tract. Since all of the substances used as a source of 

 cytost in these experiments will commonly occur in 

 animal cages, pens, and runs, it is obvious that such 

 places constitute a potential source of trouble unless 

 frequently cleaned. Obviously in crowded cages this 

 potential source of cytost is increased, due to the 

 greater number of animals per unit area. In conse- 

 quence we may conclude that the ill effects attendant 

 upon overcrowding are in part due to the greater 

 chances of exposure to air containing a relatively 



