Towards the close of the year 1872 the biological world was startled by the announcement of the discovery of minute Filariæ in human blood. Dr T. R. Lewis had found microscopic worms in the blood, and also in the urine, of persons suffering from chyluria. The worms could be obtained from day to day by simply pricking any portion of the body with a finely pointed needle. To this hæmatozoon Lewis gave the trinomial term _Filaria sanguinis hominis_, which thus fitly distinguished it from the _Filaria papillosa hæmatica canis domestici_ described by Grube and Delafond. Dr Lewis found the average size of the parasite to be 1/75″ in length by 1/3500″ in breadth. He observed that while it exists in the blood the body is enclosed in a delicate transparent tunic or cyst. The worm was never absent from urine in chyluria. In a case in which there was a milky discharge from the eyes the worms were also detected. In one case Lewis calculated that 140,000 Filariæ were present in the blood--a number certainly not relatively large seeing that MM. Grube and Delafond estimated the verminiferous blood of their several dogs to contain numbers varying from 11,000 to 224,000. Lewis also found Filariæ in the kidneys and supra-renal capsules of a woman who died of chyluria. It did not appear probable that the worms underwent further development in the human body. On this point Lewis remarks:--“Not only may those hæmatozoa found in man live for a period of more than three years, but there is no evidence that they have any tendency to develop beyond a certain stage as long as they remain in the circulation.” Dr Lewis judged that the form of chyluria associated with this condition of the blood was local and intimately related with a tropical climate. The milky condition of the urine comes on suddenly, not only at first, but on succeeding occasions also. It is frequently accompanied by more or less distinctly marked symptoms of various other obscure diseases, including temporary swellings in the face or extremities. From certain appearances of intestinal ulceration Lewis thought that the parasites might gain access to the system by the alimentary canal, possibly from the tank-water or the fish inhabiting it. He considered the state of the urine to be due to the mechanical interruption offered to the flow of the nutritive fluids of the body. The accidental aggregation of the Hæmatozoa might give rise to obstruction of the currents within the various channels, or occasion rupture of their extremely delicate walls, and thus cause the contents of the lacteals, lymphatics, or capillaries, to escape into the most conveniently placed excretory channel.