Thymectomy was performed on the urodele amphibian Pleurodeles waltlii Michah. from the 5th to the 20th week of larval development. Thymectomized animals were allografted at 7 months of age. The results obtained are as follows: (1) Pleurodeles thymectomized before 10 weeks always tolerated allografts: (2) Pleurodeles thymectomized within 11 to 13 weeks generally rejected allografts in a chronic manner, but sometimes tolerated them; (3) normal rejections occurred in Pleurodeles thymectomized at 14 weeks, or later. Stage 52 of the developmental table (10 weeks of age) is a critical "end stage" for the efficiency of thymectomy. The results are confirmed by the fate of second set allografts performed in all these animals.
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