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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



medical investigators, in addition to those already mentioned. 

 Briefly the life cycle is as follows (fig. 1) : 



Infection in a human being, pig, or other susceptible animal, 

 takes place as a result of the ingestion of meat containing live, infec- 

 tive larvae. The infective larvae, 1 mm. (about V25 of ^n inch) 

 long, are located in the muscle fibers where they are spirally rolled, 

 and enclosed in connective tissue cysts. On reaching the stomach 

 of a susceptible host that happens to ingest trichinous meat, the 



FiGDKE 1. — Life cycle of TricMnella spiralis. (After Schwartz.) 



larvae become free as a result of proteolytic enzyme activity, and 

 the liberated worms pass into the intestine with the chyme. Here 

 they become localized, at first among the folds and villi in the upper 

 part of the small intestine, and become sexually mature in the course 

 of 2 or 3 days, meanwhile increasing somewhat in size. Following 

 the mating of the worms, the females rapidly attain their growth, 

 reaching a length of 3 to 4 mm. (about Yg to Yq of an inch), bur- 

 rowing more or less deeply into the intestinal mucosa, and commonly 

 reaching the lymph spaces of the villi. The fiill-grown males, which 



