Abstract
Conclusion
1. It is clear that the cooling of chick embryos on the 9th day has made it possible for the invading organisms to increase their effectiveness in overcoming the host. This technic has the advantage of a flexible and easily controlled attenuating device. The length of time and temperature of cooling can be chosen so that various degrees of susceptibility might possibly be attained if other invading hosts were to be used. 2. However, a word of caution is needed if chick embryos are to be used in a pseudo-quantitative manner to evaluate virulence or invasive capacity of an infective agent. Considerable variation on the hatchability or vigor of the embryos may occur from one region or one strain of chickens to another(4). Also nutritional factors of the mother hen; her age; and the phase of the egg laying season(5-8), all enter into the percentage of eggs that will hatch; hence, reflect the possibility of inherited variation of the susceptibility of the host.
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