الفهرس 
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Abstract
There is a considerable interest today in birds as food producers. In Egypt, chicken industry constitutes a considerable fraction of the National agriculture income and during the last three decades, selective breeding has considerably improved food conversion rate along with increased meat and egg production however many reports suggested that massive breeding of chicken has a negative effect on the immune system. For that reason studying the primary immune system of chicken is very essential and has direct applications.
In the present study we examined the development of the primary immune organs namely bursa of Fabricius and thymus in both native (Baladi) and foreign (Hubbard) breeds through the pre-hatching (from the 8th till the 21st embryonic day) and the post- hatching phases (from time of hatching till the 8th and 13th week in both foreign and native breeds, respectively). For studying the pre-hatching phase, a total of 54 fertilized eggs (27 for each breed) were examined while 78 chicks were used to follow the post hatching development.
In both species, the development of the primary immune organs took place during the embryonic phase with rapid growth of both bursa and thymus and the appearance of active lymphocytes within their vicinities - However rate of that development was slower in the native breeds- and by the time of hatching both organs in both species were attained their definite histological structure. The organs continued to grow after hatching to reach their maximum growth by the 4th week in Hubbard species and the 6th week in Baladi one then both organs underwent physiological regression but the rate of this regression was faster in the foreign chicken than was in the native ones.
From this study it was clear that the chicken embryos had a well developed primary immune system which was able to mount an immune response through the embryonic life but this system was declined so fast in foreign breeds by the 8th week after hatching which make them more liable for infections and other stress factors when compared to the native breed.