786 PAEASITIC DISEASES. 



verted into a tape-worm, having characters quite distinct from all 

 the other tape-worms that are liable to infest the dog. About 

 eleven weeks would be necessary for the tape-worm to assume 

 its perfect or sexually mature condition. Thus, in one ex- 

 periment, where only five days were allowed to elapse before 

 the canine bearer was killed, the heads were found alive, and 

 separated from the hydatid, but displaying no trace of any 

 body. In a second experiment, where three weeks elapsed, the 

 young and sexually immature tape-worms had only attained 

 the length of one inch and a half. In a third experiment, where 

 the interval extended to two months, the tape-worms had 

 acquired a length of eighteen inches ; but the eggs, even then, 

 were not perfectly developed. 



A period of three months being somewhat more than suffi- 

 cient for the maturation of the gid tape-worm, the perfect ova 

 will by this time be found escaping from the dog along with its 

 fffices. These eggs, in common with those of other tape-worms, 

 display six-hooked embryos in their interior. Wherever the 

 infested dog wanders and passes excrement per anura, there will 

 it be privileged to distribute the ova. The eggs, if left to them- 

 selves, would do no harm ; but by various agencies they are 

 further distributed over the pastures where yearlings and sheep 

 are grazing. Millions of tape-worm germs are thus annually 

 scattered far and wide. In due course the ova are swallowed by 

 grazing animals. When the ova have arrived within the true 

 digestive stomach, the gastric juice dissolves the shells, and the 

 minute six-hooked embryos forthwith make their escape. They 

 speedily set about migrating on their own account ; and having, 

 by means of the hooks in question, bored their way into the 

 blood-vessels, they are carried to and fro in the current of the 

 circulation. By virtue of some selective capacity they seem to 

 know when they have arrived within the vessels of the brain, in 

 which organ, after escaping the vessels, they bore their way to 

 the final resting-place. Here, by a process of transformation, 

 they part with their hooks, and gradually acquire the bladder- 

 worm state, in which condition they vary in size from a pin's 

 head to that of a large walnut. To attain the perfect polycepha- 

 lous state, they require a period of about ten weeks, and thus the 

 whole cycle of development is accomplished within something 

 like five months. 



