The agent of feline pneumonitis has been grown in the allantoic cavity for 50 serial passages. During this time the amount of agent in allantoic fluid increased about 100-fold. The titer of the agent in the allantoic fluid of the individual embryo reached a peak on the 5th day and then declined. Large inocula were required in order to obtain maximal titers. The toxic factor was present in suspensions of chorioallantoic membranes in allantoic fluid from heavily infected eggs, and could be neutralized by the specific antitoxin produced in rabbits by injection of toxin in yolk sac suspensions. Electron micrographs of the agent of feline pneumonitis grown in the allantoic cavity show that the elementary body is composed of a dense centrally located substance surrounded by a thinner material, part or all of which is the limiting membrane. Separation of these two portions of the body may be due partially or entirely to distortion during drying. The wrinkled surface of the elementary bodies is evidence that such distortion does occur. The average diameter of gold-shadowed bodies is 525 mµ.
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