770 PARASITIC DISEASES. 



Dr. Davaine administered some of his five-year-old embryos 

 to rats, and had tlie satisfaction of finding a few of their eggs in 

 the faeces, with their embryos still living, and striving to get out 

 of the shells. He administered others to a cow, also introducing 

 some into the stomachs of dogs in small linen-covered flasks. As 

 a general result, it may be said that the embryos escaped from 

 their shells ; but the contents of those eggs in which the process 

 of yolk-segmentation had not arrived at the stage of embryonal 

 formation remained undigested. So far back as the year 1853 

 Verloren reared embryos in the eggs of the dog's round-worm 

 within a period of fifteen days in distilled water. Dr. Cobbold 

 has also reared the embryos of this species {Ascaris marginata) 

 in fresh water, and has kept them alive for a period of seventeen 

 months. At the expiration of this period, and during the warm 

 weather, some of them escaped from their shells. 



According to Davaine, the eggs of many nematodes will readily 

 retain their vitality though long exposed to dryness, but their 

 contents will not go on developing during this period of exposure. 

 In the case of Ascaris tetraptera of the mouse, however, embry- 

 onal formation goes on in spite of the absence of external mois- 

 ture. He has noticed the same thing in the oxyurides of rodents. 

 Dryness does not even destroy the eggs of Ascaris lumhricoides 

 and Triclioceplialus dispar. It would seem, in short, that the 

 eggs of nematodes, which normally take up their residence in 

 cats, dogs, and carnivorous aminals which reside in arid regions, 

 will develop embryos in ovo without a trace of moisture. 

 Davaine is of opinion that it is not necessary that nematode 

 embryos should pass through the body of any intermediary 

 bearer ; and he believes that they are often directly transferred 

 to the stomach of their appropriate hosts whilst adhering in the 

 condition of an impalpable dust to the coats of their bearers, 

 whence they are detached by the animals themselves when 

 licking the fur. With the eggs of Ascaris megaloccphala Dr. 

 Cobbold has performed several experiments, having reared the 

 embryos in simple fresh water, and found them capable of escap- 

 ing from their shells dviring warm weather. He also succeeded 

 in rearing these larvse in pond mud, noticing at the same time 

 that after their exclusion they grew more or less rapidly up 

 to a certain point, after which they appeared to stop, as 

 if waitin"; transference to some host for the further accom- 



