[Illustration: FIG. 109.--Spores of _Sarcocystis tenella_, Raill. _a_, fresh, showing the polar capsule; _b_, stained, showing metachromatic granules and nucleus. × 1,000. (After Laveran and Mesnil.)] The gymnospores of _Sarcocystis muris_, from the mouse, show active boring movements when kept in saline solution warmed to 35° or 37° C. _S. muris_ is very deadly to its host. From their structure the spores do not appear to have great powers of resistance to external conditions. They measure 12 µ by 3 µ to 4 µ or less. Laveran and Mesnil (1899) isolated a toxin from _S. tenella_ of the sheep and called it sarcocystin. This substance is especially pathogenic to experimental rabbits. The duration of life of the Sarcosporidia is a comparatively long one. The affected muscular fibres may remain intact and capable of performing their functions for a long time, but at last they perish, if the host lives long enough. Thus the Sarcosporidia of the muscles are then enveloped only by sarcolemma, and finally, when this likewise disappears, they fall into the intra-muscular connective tissue. In many cases the Sarcosporidia die off within their hosts, this, according to Bertram, being brought about by a disintegration of the spores in the central chambers. In other cases the leucocytes play a part in the destruction of the Sarcosporidia, and sometimes it happens that lime salts are deposited in and around the vacant cylinders. In some places pigs, sheep, mice and rats are infected with sarcosporidiosis to a remarkable extent, in certain cases almost reaching 100 per cent. Young animals also are infected, and perhaps infection only takes place during youth. Although the natural mode of transmission of the Sarcosporidia remains to be determined, yet various experimental researches on the problem are of interest and importance. Theobald Smith (1901) found that mice could be experimentally infected with _S. muris_ by feeding them with the flesh of other infected mice. The incubation period was a long one, namely forty to fifty days. Thus, on the forty-fifth day after feeding young Sarcosporidia were found, and seventy days after feeding spore formation began. Ripe spores were found two and a half to three months after the commencement of these experiments. This mode of infection--a cannibalistic one--hardly seems likely to be the natural method for the infection of sheep and ruminants generally. Smith’s researches have been confirmed. Nègre[225] (1910) found that the fæces of mice fed on infected muscular tissue were infective to other mice when ingested by them. Negri[226] infected guinea-pigs with _S. muris_ by feeding them on infected mouse flesh, and found that the parasite in guinea-pigs showed different characters from those exhibited by it in mice. Darling[227] also succeeded in infecting guinea-pigs with _S. muris_, and Erdmann infected mice with _S. tenella_ (from the sheep). [225] _C. R. Soc. Biol._, lxviii, p. 997.