324 



ENTOZOA. 



main with the conclusions deducible from numerous recent experi- 

 ments, made with the view of ascertaining the development and 

 migrations of the Nematoda. Thus, for example (as I have else- 

 where remarked), in the case of the closely allied Ascaris lum- 

 bricoides, the independent investigations of Eichter and Davaine, 

 with fresh eggs, " only go to prove that after the ova have escaped 

 passively, jjer vias naturales, they complete their embryonic 

 development whilst free in open waters. In Eichter's experiment 

 none of the embryos had emerged from the eggs, although they 

 had been in water eleven months ; whilst the previous investiga- 

 tions of Yerloren with the eggs of Ascaris marginata of the dog, 

 showed that the young embryos can retain their vitality for more 

 than a year after their worm-like condition has been attained." 

 After a similar manner the escaped ova of IricJiocephalus dispar 

 require a period of six months after their expulsion before the 

 embryonic condition is fully attained ; but in the case of Ascaris 

 osculata infesting the seal, my own recent experiments go to prove 

 that the embryonic development may be completed a few days after 

 the eggs have escaped into open waters. On the whole, therefore, 

 it has become evident that some round worms pass through their 

 early development much quicker than others ; and in the case of 

 those which do not produce their young viviparously, it is neces- 

 sary that the ova become immersed in water for a longer or shorter 

 interval ; and it is chiefly through this aqueous medium that they 

 gain access to our bodies. Before very long, we hope to be able to 

 give a complete record of the history of the development of each 

 individual species which infests the human body ; but those who 

 have not practically attended to these inquiries can scarcely be 

 aware of the numerous difficulties which beset our investigations 

 in this direction. 



Leaving now, therefore, the question as to the mode of immi- 

 gration to the human host, it remains for me to direct more 

 particular attention to the specific characters of Ascaris mystax. 

 The accompanying figures, with their explanatory references below, 



