The production and the circulation of lymphocytes has been examined in the sheep fetus where neither foreign antigen nor immunoglobulins occur. It was found that as the lymphoid organs increased in size during fetal life, the numbers and the output of lymphocytes in the thoracic duct lymph increased. The recirculating pool of lymphocytes was estimated to be 5.5 +/- 1.5 X 10(8) cells in fetal lambs 95-100 days of age, 5.7 +/- 1.2 X 10(9) cells in fetuses 130-135 days of age, and 1.2 +/0 9.3 X 10(10) cells in fetuses near to term. The rate of addition of lymphocytes to the recirculating pool was 3.2 +/- 1.9 X 10(6) cells/h in fetuses of 100 days and 3.4 +/- 0.9 X 10(7) cells/h in fetuses of 130 days of age. Lymphocytes recirculated from blood to lymph in fetuses; labeled cells injected into the blood stream reappeared in the thoracic duct lymph promptly and reached maximum levels around 12-18 h after they were injected. Labeled lymphocytes were detected subsequently in greatest numbers in the lymph nodes, particularly in the mesenteric lymph nodes and in the interfollicular areas of the Peyer's patches. Chronic drainage of thoracic duct lymph from fetuses in utero for periods of up to 36 days had no obvious effects on the growth or development of the fetus and only minimal effects on the content of lymphocytes in the various lymphoid tissues even though the number of cells in the blood and lymph were reduced to between 20-30% of normal levels. Thymectomy done in fetuses about 2 mo befor cannulation of the thoracic duct reduced the output of cells in the thoracic duct to about 25% of normal levels and caused a significant reduction in the content of lymphocytes in the various lymphoid tissues. Thymectomized fetal lambs subjected to thoracic duct drainage for periods up to 2 wk in utero had a similar complement of lymphocytes in their lymphoid tissues to intact thymectomized fetal lambs. Lymphocytes obtained from the thoracic duct lymph of lambs thymectomized 2 mo previously recirculated from blood to lymph when they were injected intravenously, although they did this at a significantly slower rate than did lymphocytes from normal lambs.

SUMMARYB-cell depleted fetal sheep were created following a single injection of an anti-IgM monoclonal antibody early in gestation. Six sheep fetuses were given a single intraperitoneal injection of a monoclonal antibody directed against IgM at 63 days of gestation (gestation in sheep 150 days). The fetuses were killed at 138-142 days of gestation and lymphoid tissues were collected for subsequent light microscopy and immunohistochemical examination. The ileal and jejunal Peyer's patch (PP) follicles in four of the six injected fetuses were markedly reduced in size. Cells in the rudimentary follicles of the ileal PP of these animals showed no reactivity for IgM and most were negative for CD45. The dome regions contained many T cells, which were predominantly CD8 cells and included

Summiry. The intestine of the foetal lamb was exposed to large quantities of V-globulin (IgG) by prolonged intra-duodenal infusion, and absorption of intact IgG, with transfer to the lymph, continued undiminished. i.e-, there was no evidence of closure. The rate of proliferation of the intestinal epithelium of the foetal and newborn lamb was measured using mitotii: indices and localised labelling wiih ('H)-thymidine (TdR). In the foetus, cell division in the crypts occurs at a lower rate than the newborn (p<0 (X)l) and there is very slow replacement of ihe intestinal epithelium. In the newborn lamb, a portion of the small intestine was incubated in vivo with TdR and the progress of labelled cells from the crypts upwards along the vitii estimated, using autoradiography of serial biopsies from the same animal. A front of mature, digestive epithelium could be seen advancing up the villi, displacing the immature foetal lype of cell which was capable of transfer of intacl IgG to the lymphatics of the intestine. The evidence presented supports the hypothesis that immediately after birth the intestinal epithelium of the lamb begins to be replaced by a digestive type of cell, and the layer of eells responsible for absorption of cotostral antibodies progressively disappears from the villi, resulting in closure.

A competitive binding technique is described for the estimation of the thyroxine (T4)-binding properties of serum proteins in dilute blood serum and lymph. When used in conjunction with an assay for total T4 the following parameters can be estimated: the number of functionally different T4 binding proteins, their individual association constants and binding capacities for T4, the amount of T4 which is bound to each binding species, and the concentration of unbound (free) T4. Both human and sheep serum have three functionally different T4-binding proteins. The association constants for the three human proteins were 9.5x10(9), 1.6x10(8) and 3.1x10(5) 1/mol for T4-binding globulin (TBG), T4-binding prealbumin (TBPA) and serum albumin, respectively. The corresponding sheep proteins, TBG, TBP-2 and albumin, had association constants of 8.9x10(9), 1-4 X 10(8) and 3.5x10(5) 1/mol. Human TBG had a mean binding capacity of 21.3 mug/100 ml and that of ovie TBG was 12.8 mug/100 ml. The other specific binding proteins (TBPA in man and TBP-2 in sheep) had mean binding capacities of 307 and 359 mug/100 ml respectively. Two functionally different T4-binding proteins were identified in rat serum.

The oxidation of 14C-labelled compounds has been studied by administering these compounds to animals and subsequently collecting and analysing their expired CO2 for radioactivity. When an attempt is made to relate the appearance of radioactivity in the expired breath with the rate of oxidation of the specific labelled compound, several difficulties arise in the interpretation of the patterns of 14CO2 excretion. This is due to the multiplicity of metabolic events concerned in the conversion of a metabolite to C02, and in the subsequent transport and elimination of this CO2 from the body.