ANIMAL BREEDING 199 



higher concentration of cytost than would be the case 

 if fewer animals were confined within the same area. 

 Further, it is possible that cytost finds its way into the 

 air of the cage by way of the effluvia of the breath. 



Further evidence that in crowded cages cytost may 

 be present in sufficient quantity to elicit various respi- 

 ratory disturbances was obtained as follows : Pregnant 

 animals were allowed to give birth to their young in 

 cages which had previously been inhabited by a num- 

 ber of adult animals. Under such conditions it was 

 found that the newly born offspring usually died 

 within two weeks, from respiratory disturbances. 



The results of experiments of this type confirm the 

 hypothesis that the ill effects of overcrowding are due 

 in part to the resultant relatively high concentration 

 of cytost which accumulates in the cages or pens. 



In raising small animals in the laboratory, it almost 

 inevitably results that a certain degree of crowding 

 is necessitated by the requirements of space. This con- 

 dition should not present any serious difficulties if the 

 cages are cleaned thoroughly at regular intervals, 

 preferably daily. Such a cleaning should involve not 

 only the removal of excreta and the like, but the com- 

 plete removal of hair and dust. As a further means of 

 minimizing the dangers of contact with cytost, the 

 rooms in which the animals are kept in their cages 

 should be well ventilated, as this will prevent the ac- 

 cumulation of cytost-laden air. 



This fact has been appreciated by the laboratory 

 workers engaged in researches on the white rat, and 

 a visit to laboratories where this animal is raised in 

 large numbers will show that the conditions stated 

 above are scrupulously observed. For some curious 



