186 



PARASITISM 



of a muscle fiber (Fig. 79), in which condition it is known as Miescher's 

 tube, a name apphed to the vegetative forms of the mouse parasite 

 Sarcocystis muris. As the young trophozoites grow, the nuclei increase 

 in number, a definite sac-like membrane develops around the proto- 

 plasmic body, while in the centre groups of spores begin to form. 

 The ripening spores (merozoites, gymnospores) gradually encroach 

 upon the more peripheral unused protoplasm of the tube until the 

 ends only appear to be active, and capable of vegetative functioning, 

 and even these, finally, are used in spore formation. In Sarcocystis 

 tenella of sheep such cysts may grow to a length of two inches in the 

 muscle bundles, where they ultimately burst, the spores being scattered 

 or carried by blood to new regions, where development begins anew 

 (auto-infection). In some cases the entire body may be over-run by 

 such parasites, mice especially often being killed in this manner. 



Fig. 79 



Sarcocystis minis, a muscle parasite of the mouse. (After Jlinchin.) 



In all cases there is every reason to believe that this method of 

 endogenous multiplication cannot be continued indefinitely any more 

 than a paramecium can continue to divide indefinitely, and there is 

 reason to suppose that the potential of vitalit}' gives out at the end of a 

 more or less definite cycle of generations. In many cases, especially in 

 the disease-causing forms in man, the organisms seem to have devised 

 a means of counteracting this senile process and of being stimulated 

 to renewed activity in much tlie same way that paramecium was 

 stimulated by artificial means (see page 131). It is a recognized fact 

 that many of the blood diseases are characterized by relapses in which 

 the organisms reappear after having disappeared from the circulation. 



