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The ileocecal valve may also bar the spread of tumor cells to a certain extent, since in eight cases

Sixteen cases of an entity, periodic neutropenia, have been collected. They are characterized by remarkable clinical uniformity and regular recurrences of neutropenia at three week intervals. The entity may be a variant of a larger group of periodic conditions. The cause is unknown.

Variations in the agglutinability and virulence of certain strains of pneumococci have long been noticed.Neufeld (1) reported in 1902 that a strain which had previously been agglutinated by his immune serum, after it had been grown for a long time in artificial media, failed to be agglutinated. At the same time it had become avirulent.In more recent years much work has been carried on concerning the specificity of the various types of pneumococci. In this work the strains chiefly studied have been those freshly isolated from the human body and the constancy of the immunological specificity of these strains over prolonged periods of time has seemed truly remarkable. For instance, a strain has been continuously cultivated at the Hospital of The Rockefeller Institute for 14 years and has retained its type specificity unaltered under the conditions prevailing in the laboratory. However, attention has been drawn from time to time to the possibility of modifications in specificity occurring when certain strains are grown under unusual conditions. The observation of Neufeld already mentioned and a similar finding by Cotoni (2) in 1912 suggested such an occurrence. In 1915 Friel (3) found that when pncumococci were grown in immune serum they became less virulent and became agglutinable and phagocytable in normal rabbit serum. About the same time Stryker (4) showed in this laboratory that when a specific virulent strain was grown in homologous immune serum it lost its specific agglutinability, the virulence was decreased, and the power to produce capsules was inhibited. She found that by passage through a few animals the strain regained its original properties. However, she was not studying strains derived from a single cell. More recently Yoshioka (5) has drawn attention to the fact that when pneumococci are grown under certain unusual conditions, as on unfavorable media, or at 39°C., or when cultures are allowed to undergo drying, variations in serological reactions appear. The changes noted were a decrease in agglutinability in homologous serum and the appearance of a tendency to agglutinate in heterologous serum. At the same time the strains became less virulent. He also found that these modifications did not appear simultaneously in all the bacteria of the cultures, but that when cultures undergoing modifications were plated, the bacteria from certain colonies showed 587 on