GENERATION OF NEMATOID WORMS. 161 



some sewer or cesspool, where they remain till they are conveyed 

 to the fields in the shape of manure. From this point it is 

 difficult to trace their course with certainty, but it is believed 

 that they have the power of supporting their vitality till they 

 reach some collection of water by the agency of rain and the 

 ordinary drains of our fields. In 1853, Verloren placed a fragment 

 of a mature female Ascaris marginata from the dog in water, 

 examining the eggs from time to time. The development of the 

 young immediately commenced, and in about fourteen days was 

 completed, perfectly developed young worms making their ap- 

 pearance within the egg-shells, but not breaking them, although 

 kept in different temperatures for more than a year. This fact 

 is very remarkable, being in direct opposition to the ordinary 

 course of nature, which usually provides for the preservation of 

 ova under many vicissitudes of heat and moisture, but when 

 once the animal begins to be developed, its progress must be 

 steady or it dies. Here, however, the developed embryo is 

 arrested in its shell, and becomes, as it were, torpid, till it reaches 

 its proper habitat, the intestine of the dog who has swallowed the 

 water in which it has been suspended. 



Such is a brief summary of our knowledge (much of which is 

 hypothetical) of the generation of these worms, and it will be 

 apparent by comparison with the natural history of the Tcenice, that 

 less is known of the round-worm than of the tape-worm. Still I 

 think it may be accepted as true in the main, or sufficiently so to 

 put us on our guard against the chief means by which these worms 

 enter and infest the inmates of our kennels. How the ova find 



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