1. The development of adult duodenal antigens in the embryonic duodenum was studied by means of the Ouchterlony agar diffusion technique.

  2. In 11-, 12-, and 13-day embryonic duodena there were two antigens; by the 14th day three antigens; in the 15-, 16-, and 17-day duodena four antigens; while in 18-, 19-, and 20-day duodena there were only three antigens with a fourth antigen again present in the 21-day duodenum. The disappearance of the fourth antigen found in 15-, 16-, and 17-day duodena was attributed to a change in molecular configuration of the antigen in the process of differentiation.

  3. Following grafts of adult duodenum there was an increase of three antigens observed in the host duodenum and an increase of one antigen in the host spleen.

  4. When adult spleen was grafted there was no increase in the number of antigens in the host spleen, but an increase of one antigen was observed in the duodenum.

  5. The results are discussed in the light of the organ-specific growth stimulation hypothesis, but this is rejected because it cannot account for the generalized effects observed.

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