The only cause of this disease is the reception into the system of the embryo of the tape-worm. These embryos are only to be found in the flesh of other animals. The principal sources of human infection are beef and pork. The embryos are found in the muscular tissue, or lean meat, inclosed in little cysts. In Figs. 279 and 280 the embryo of the tape-worm, known as cysticercus, is shown of natural size and slightly magnified. Plate IX shows the head of an embryo such as is found in the flesh of the hog, greatly magnified. When flesh containing the embryos is eaten, the cyst is digested off by the gastric juice, and the embryo attaches itself to the mucous membrane of the small intestine, by means of its hooks and suckers. In a short time a small body is formed, which is quickly duplicated, and the process continues until from an insignificant beginning the formidable length of fifteen, twenty, and even forty or fifty, or more, feet is formed. Thus the worm, when fully developed, is really a chain of living creatures, each link being a separate individual, producing eggs in vast numbers, which pass out of the body in the discharges, and, finding entrance into the stomach of some other animal, develop into embryos, to be again eaten by man, or some other animal, in whom the fully developed worm will be produced.

Fig. 279. Small Tape-Worm Embryo.

Fig. 279. Small Tape-Worm Embryo.

Fig. 280. Large Tape-Worm Embryo.

Fig. 280. Large Tape-Worm Embryo.