(3) The afore-mentioned views were criticized, and the bodies were not considered to be living organisms but merely reaction products or cell-inclusions due to the effects of the virus on the host cells. Thus Guarnieri’s bodies were stated to consist of extruded nucleolar or plastin material, having no developmental cycle. It was further asserted that infection could be produced by lymph in which Guarnieri’s bodies had been destroyed. Similar assertions have been made regarding the Negri bodies, and others. The _Cytoryctes_, _Neuroryctes_, etc., are considered, according to these views, to be degeneration products of the nucleus or to be of a mucoid nature. (4) More recently a positive belief has gained ground that there are true parasitic organisms causing these diseases, and that the parasites are very minute, being termed Chlamydozoa by Prowazek and Strongyloplasmata by Lipschütz. The Chlamydozoa are characterized by (_a_) their very minute size, smaller than any bacteria, so that they can pass through bacterial filters; (_b_) they pass through intracellular stages, in the cytoplasm or the nucleus of the host cell, producing therein the reaction products or inclusions in the cell already recorded as characteristic or diagnostic of the diseases produced; (_c_) they pass through definite developmental cycles. Such a cycle consists essentially of growth and division. The mode of division of the Chlamydozoa resembles that of the centriole of a cell, by the formation of a dumb-bell-shaped figure. Two dots are observed connected by a fine line or strand which becomes drawn out and finally snaps across the middle. Prowazek and Aragão (1909) working on smallpox in Rio de Janeiro found that the chlamydozoal granules passed through a Berkefeld filter and that the filtrate was virulent. But if an “ultra-filter” were used, _i.e._, one coated with agar, then the granules were retained and the filtrate was no longer virulent. The surface of the ultra-filter was found to contain many granules. The Chlamydozoa are parasites of epiblastic tissues (_e.g._, epidermal cells, nerve cells, conjunctival cells). [Illustration: FIG. 119.--Chlamydozoa. Trachoma bodies in infected epithelial cells of the conjunctiva. (_a_) initial bodies (above) and cluster of elementary bodies (touching the nucleus); (_b_) cluster of granules surrounded by mantles. × 2,000 approx. (Original. From preparation by Fantham.)] The life-history of a Chlamydozoön (fig. 119), such as that of vaccinia, is, according to Prowazek, Hartmann and their school, as follows:-- 1. The infection begins with _elementary bodies_ or _elementary corpuscles_ which live at first extracellularly. An elementary body is a minute speck of chromatin, apparently devoid of cytoplasm, which can pass through a bacterial filter. It can enter a host cell, but the entry is not a process of phagocytosis. 2. Inside the host cell the elementary body grows in size, and becomes an _initial body_ (fig. 119, _a_).