Under this generic term we comprehend such formations, infesting the organism both within and without, as represent independent entities,, either from the vegetable or animal kingdom. Their investigation belongs to pathological anatomy in general, but especially so, inasmuch as the presence of parasites not only implies previous, but also engenders new, morbid conditions. Moreover, they merit a place in this chapter because-it is daily becoming more clear that they are not the production of a generatio cequivoca out of diseased organic matter, but that they enter into the organism from without, and find there a soil appropriate for their subsistence and growth.

Parasites are introduced into the organism either as seeds, as ova, or in a more advanced condition, - to germinate, become developed, or grow, in or upon the organism. Nor is it less evident that certain pathological states determine a disposition, not exactly to the generation, but to the evolution and redundant growth of parasites, for which they furnish the necessary conditions. Thus, parasite plants (fungi) readily and commonly germinate in particular exudates upon mucous membranes, whilst upon normal mucous membranes their sporules remain undeveloped. Parasites become pernicious in various ways.