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XLV] within the cirrus pouch. The gravid uterus occupies nearly the entire segment. The eggs number from 80 to 180 in each proglottis; they are oval or globular, and present two distinct membranes; the outer one measures from 30 to 60 μ in diameter, the inner one from 16 to 34 μ. The latter exhibits at each pole a more or less conspicuous mammillate projection.

Development.—The development of Hymenolepis nana has been worked out experimentally by Grassi in the rat. The egg is swallowed, and after hatching enters a villus in the lower part of the small intestine (Fig. 200), where it transforms into a cercocystis. The six-hooked embryo is found concysted from 24 to 50 hours after ingestion, its long axis corresponding to that of the villus, and the posterior end bearing the embryonal hooks directed towards the lumen of the intestine. Occasionally two parasites may be found in the same villus. In about 40 to 70 hours after ingestion the scolex has appeared, and in 80 to 90 hours after ingestion the rostellum is

provided with booklets. Then the parasite passes into the lumen of the intestine, where it can be seen attached to the epithelium of the villus with short neck and no trace of segmentation. The rapidity of development varies somewhat, and, as a rule, various stages are found occurring simultaneously in the same host. Strobilization is rapid; the proglottides attain maturity in about 10 or 12 days, and about 30 days after infection the eggs of the parasite begin to appear in the fæces. These never develop in the same host unless