Jan. IS, 1920 Further Studies of Sorosporella uvella 421 



The theory set forth by Metchnikoff {14) stated that when an animal 

 is attacked by a hostile organism its blood corpuscles and other proli- 

 ferating cells of mesodermic origin ingest and destroy the parasite. For 

 a number of years this theory together with the Humoral theory have 

 been considered the two most plausible explanations of immunity ; but 

 unfortunately each attempted to explain this phenomenon from a point 

 of view that seemed to be directly opposed to the other, the Humoral 

 theory holding that hostile organisms were destroyed by a body fluid 

 in the nature of a serous exudate. 



These differences appear to have been somewhat adjusted in recent 

 years by the work of Erlich, Denys and Leclef, Wright, and others, which 

 has shown the body fluids and the phagocytes to be interdependent in 

 certain human diseases. Wright, notably, proved that the act of pha- 

 gocytosis is dependent, at least in some diseases, upon the presence of 

 certain substances in the blood, and that these substances (opsonins) act 

 upon the invading organisms, not necessarily killing them, but rendering 

 them susceptible to ingestion by the leucocytes. 



It is believed by a certain group of men at the present time that the 

 act of phagocytosis is directly associated with the process of immuniza- 

 tion, although the destruction of hostile organisms is brought about not 

 by the phagocytes alone but with the aid of certain body fluids, like 

 opsonins, the nature of which is not fully understood at the present 

 time. Other factors probably are important, such as the potency of 

 the parasite; but these can not be considered here. 



While it has been shown in certain human diseases that the leucocytes 

 actually ingest parasitic organisms in a manner comparable to that 

 described above, it is nevertheless well known that such ingestion may 

 end in the destruction of the phagocytes as well as in the death of the 

 organisms, the result apparently depending upon the relative resistance 

 or potency of the leucocyte and parasite. 



A study of the blood of infected cutworms has shown that the blasto- 

 cysts of the parasitic fungus occur within the cytoplasm of the leucocytes. 

 The process by which this position is obtained has not been observed in 

 living material, but from a study of stained individuals it is clearly 

 indicated that amoeba-like pseudopods arise from the phagocytes by 

 means of which the blastocysts are incorporated into the substance of 

 the leucocyte. The occurrence of the phagocytes in aggregations or 

 cysts, however, indicates that some sort of attraction exists between 

 the insect cells and the fungus cells. It is well known that mobile 

 protoplasmic cells exhibit certain definite movements when subjected to 

 mechanical, chemical, or thermal stimuli; and in view of the discovery 

 of Wright, it seems likely that some substance is present in the blood 

 plasm of infected cutworms which not only renders the blastocysts 

 susceptible to ingestion but which also may exert some sort of attrac- 

 tion, perhaps chemotactic, upon the leucocytes, rendering the formation 



