Summary. The effects of in utero thymectomy on the suhsetiuent growth rate, the levels of circulating white blood cells, the levels of lymphocytes in lymph and the .size and development ot the peripheral lymphoid orjjans have lieen invcstipjiU'd in lambs. Consistent and severe depressions were observed in the circiiliitin^ lymphocyte counts in both the blood and the lymph of thyniectomized lambs of 3 months of age; hy 10 months of age, althoii];:h these levels were still significantly below normal, the differences in the circnliitinl ymphocyte count.s \vere much less.Histotouical o!>servations .showed that the Iymphuid tissues were depleted of cells to viirjing degrees. 7iic tissues of lambs tliymectoiiiized at 80-^)0 day.s I)o.st-conct'ption were generally less depleted than lanil)s whiuli were thymectomized at 60-70 days post-conc'eption, The "wasting syiitlroiue" descnlied to occur following thymectomy in mice was not seen in lambs.Tlie.se results in the lamb snggest that, while thu tliynuis is a source of lymphocytes, it is not the only source and a eimsiderable proportion of the lymphocytes present in normal animals is not derived directly from the thymus.

Summary Surgical techniques are described for establishing chronic fistulae in the thoracic, intestinal, lumbar and popliteal lymph ducts of foetal lambs in utero. These fistulae have remained patent and enabled lymph to be collected for periods of up to 5 days. The Iambs have been born spontaneously or delivered by Caesarean section with their fistulae intact and still flowing. Lymph flow rates were 9–18 ml./hour from the thoracic duct, 2.8–8.6 ml. /hour from the intestinal duct, 2.8–9.2 ml./hour from the lumbar trunk and 0.6 ml./hour from the popliteal duct. The protein concentration of foetal thoracic duct lymph was 2.30 g./100 ml. immediately after cannulation, the intestinal lymph 2.37 g./100 ml. and the lumbar lymph 2.12 g./100 ml. The cells of foetal lymph were almost all small and medium lymphocytes and only an occasional cell was found in mitosis. Drainage of the thoracic duct lymph for 36 hours drastically reduced the number of lymphocytes in the lymph, suggesting that, as in the adult sheep, most of the lymphocytes in foetal lymph are recirculating cells.

ABSTRACT

Summary. Laiiihs thyiiit'ttotiiizt'd in utero prior Ui the 80l!i day nf gestation li;iM' iu'fii shown to hf capable of rejecting grafts of allogeneic skin as vigorously us normal lambs, even when grafted as long as 21 iiion(hs after tliyinectomy. Hi.itologically, the cellular reactions involved were the same in thymectomized lambs as in contn)l lanihs. Since these lanihs were tliyniectoinized at an age at which it has been .sliown by other workers that lanibs are iuia))le to reject allogeneic skin grafts, it was i-ontliitled that the thymus plays no e.'isential part in tlie final development ur maiiitfiianct' of this particular immune reaction in sheep.Thymectomized lambs were also found to bt* fully capable of motuiting an iuimediate type hypersensitivity re.sponse to ferritin. although the delayed type hypersensitivity response to tuln?rculin was severely reduced. Since it has t)een shown that footal lambs are incapable of re.sponding to BCt; at any time before I)irth. it is po.ssihle that the thymus is a source, although not the sole source, of cells which take part in delayed tyjjt hyperst'nsitivity responses.

Summary. Tiie responses of in ulcro thymectomized lambs to three different antigens, .swine influenza virus (strain Shope 15), boiled SaUmmella muenchetx organisms and chicken red blootl cells, have been studied in terms of the cellular and antibody responses in lymph coming from the popliteal node.It was found that, although thymectomized lambs had a greatly reduced total cell output from tlm popliteal node, the significant features of the immune response in these animals were unaffected. There were no significant differences between thymectomized and control lambs in temis of the production of large cells or basophilic celLf, neither were there any diHerences in liie uumber of plaque-forming cells leaving the node, in total antibody production iti lymph or sera, nor any qualitative differences in tlie type of antibody produced.