LARYNGOTRACHEITIS VACCINATION 

 Table 2. A Comparative Classification of Takes 



375 



Invisible Takes 



It should be noted in Tables 1 and 2 that there were 21 birds in which no 

 visible takes appeared. Since the bursa of Fabricius had completely involuted 

 in the hens and roosters and wad represented only by a bursal fold, the 10 birds 

 not reacting to the vaccination in this group may have been naturally resistant 

 to the disease. Ju^t what part natural resistance plays in infectious laryngo- 

 tracheitis has not been fully determined, although there is some evidence to 

 indicate that it may be an important factor in baby chicks receiving nourishment 

 from the yolk-sac and adult birds eight months or older. 



In this experiment 200 chickens corresponding to the i-3 and the 4-7 months 

 old groups recorded in Tables 1 and 2 were vaccinated. The chickens were 

 killed on the fifth day following vaccination, and necropsied for evidence of takes, 

 special attention being pa'd to the bursa of Fabricius. The results are given 

 in Table 3. 



This table shows that six takes invisible to the eye occurred in the bursa of 

 Fabricius in the first group, and three in the second. These numbers correspond 

 approximately to the numbers eight and three indicating no visible takes in 

 Tables 1 and 2. 



Table 3. A Comparative Study of Takes in the Bursa of 

 Fabricius and Cloaca. 



