In the youngest Sarcosporidia (40 µ in length) from the muscles of the sheep there occur, according to Bertram, small roundish or oval cells (4 µ to 5 µ), the nuclei of which are half their size, and are embedded in a granular protoplasmic mass. In somewhat larger, and therefore older, cylinders, the investing membrane of which already shows both layers, the cells have become larger (to 7 µ) and are more sharply outlined from each other (fig. 106). These uninucleate cells may be considered as pansporoblasts. In each pansporoblast division of the nucleus occurs (fig. 107), and meanwhile the pansporoblasts become isolated within the chambers, the dividing partitions of which originate from the granular protoplasm which is present between the pansporoblasts. The numerous uninucleate daughter forms produced within the chambers become spores direct (fig. 108). The process commences in the centre of the cylinders or sarcocysts, and then progresses towards the extremities, the parasites meanwhile increasing in size, and new pansporoblasts being continually formed at the extremities (fig. 107). [Illustration: FIG. 105.--Transverse section of _Sarcocystis tenella_, Raill. From the œsophagus of the sheep, _Ovis aries_. × 38. _a_, marginal chambers filled with spores; _b_, connective tissue of the œsophagus; _c_, muscles of the œsophagus.] [Illustration: FIG. 106.--Young _Sarcocystis tenella_ of the sheep, 47 µ in length. (After Bertram.)] [Illustration: FIG. 107.--End of a trophozoite of _Sarcocystis miescheriana_ from the diaphragm of the pig, showing division in pansporoblasts. × 800. (After Bertram.)] [Illustration: FIG. 108.--_Sarcocystis blanchardi_ of the ox. Longitudinal section of sarcocyst or Miescher’s tube. _a_, substance of muscle fibre; _b_, envelope of sarcocyst; _c_, muscle nuclei; _d_, spores in chambers; _e_, ground substance. × 400. (From Wasielewski, after van Eecke.)] The spores (sometimes called Rainey’s corpuscles), vary in shape according to the species, but are also of different form individually. They are mostly kidney-, bean- or sickle-shaped (fig. 109), and of small size, sometimes reaching 14 µ by 3 µ to 5 µ. They are apparently surrounded by a thin membrane, and at one extremity (according to the discovery of L. Pfeiffer, confirmed by van Eecke, Laveran and Mesnil) contain an obliquely striated body (fig. 109) often homologized with the polar capsule, while the greater part of the spore is taken up by the nucleate sporozoite. Several authors state that they have also observed filamentous appendages (polar filaments) at one end of the spores, and have seen two kinds of spores in the same Sarcosporidium. Spores of various species of Sarcosporidia may contain metachromatic granules, often centrally placed (fig. 109). These granules may be metabolic or possibly may contain toxin (see below).