This report has attempted to analyze the changes in the absolute number of small lymphocytes in the blood stream of the cat following the intrathoracic ligation of the thoracic duct. Such a ligation produced an immediate decrease in the number of small lymphocytes to the extent of 56 per cent, but it was found that the preoperative level was again reached at about the end of 3 weeks. One is led to believe that the gradual return of the number of small lymphocytes to the preligation level took place pari passu with the establishment of the collateral circulation of the thoracic duct, although there is no absolute proof of this. Yet it is definite that the thoracic duct is an important avenue for the entrance of small lymphocytes into the blood stream, and that it is the pathway through which at least half of the small lymphocytes reach the circulating blood in the cat.

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